Backends are registered to the BackendService through new data
structures:

Backends are concrete classes, deriving from
\OCA\Files_External\Lib\Backend\Backend. During construction, the
various configuration parameters of the Backend can be set, in a design
similar to Symfony Console.

DefinitionParameter stores a parameter configuration for an external
storage: name of parameter, human-readable name, type of parameter
(text, password, hidden, checkbox), flags (optional or not).

Storages in the StoragesController now get their parameters validated
server-side (fixes a TODO).

UserGlobalStoragesService reads the global storage configuration,
cherry-picking storages applicable to a user. Writing storages through
this service is forbidden, on punishment of throwing an exception.

As UserGlobalStoragesService and UserStoragesService share a bit of code
relating to users, that has been split into UserTrait. UserTrait also
allows for the user set to be overridden, rather than using the user
from IUserSession.

Config\ConfigAdapter has been reworked to use UserStoragesService and
UserGlobalStoragesService instead of
OC_Mount_Config::getAbsoluteMountPoints(), further reducing dependance
on that horrible static class.

A backend can now specify generic authentication schemes that it
supports, instead of specifying the parameters for its authentication
method directly. This allows multiple authentication mechanisms to be
implemented for a single scheme, providing altered functionality.

This commit introduces the backend framework for this feature, and so at
this point the UI will be broken as the frontend does not specify the
required information.

Terminology:
 - authentication scheme
    Parameter interface for the authentication method. A backend
    supporting the 'password' scheme accepts two parameters, 'user' and
    'password'.
 - authentication mechanism
    Specific mechanism implementing a scheme. Basic mechanisms may
    forward configuration options directly to the backend, more advanced
    ones may lookup parameters or retrieve them from the session

New dropdown selector for external storage configurations to select the
authentication mechanism to be used.

Authentication mechanisms can have visibilities, just like backends.
The API was extended too to make it easier to add/remove visibilities.
In addition, the concept of 'allowed visibility' has been introduced, so
a backend/auth mechanism can force a maximum visibility level (e.g.
Local storage type) that cannot be overridden by configuration in the
web UI.

An authentication mechanism is a fully instantiated implementation. This
allows an implementation to have dependencies injected into it, e.g. an
\OCP\IDB for database operations.

When a StorageConfig is being prepared for mounting, the authentication
mechanism implementation has manipulateStorage() called,
which inserts the relevant authentication method options into the
storage ready for mounting.

Prior to this, the storage class name was stored in mount.json under the
"class" parameter, and the auth mechanism class name under the
"authMechanism" parameter. This decouples the class name from the
identifier used to retrieve the backend or auth mechanism.

Now, backends/auth mechanisms have a unique identifier, which is saved in
the "backend" or "authMechanism" parameter in mount.json respectively.
An identifier is considered unique for the object it references, but the
underlying class may change (e.g. files_external gets pulled into core
and namespaces are modified).

The following functions have been removed:
 - addMountPoint()
 - removeMountPoint()
 - movePersonalMountPoint()
 - getSystemMountPoints()
 - getUserMountPoints()

registerBackend() has been rewritten as a shim around BackendService,
allowing legacy code to interact with the new API seamlessly

addMountPoint() was already disconnected from all production code, so
this commit completes the job and removes the function itself, along
with disconnecting and removing related functions. Unit tests have
likewise been removed.

getAbsoluteMountPoints() has been rewritten to use the StoragesServices

The same app container must be used in the settings templates as used in
OC_Mount_Config, so that any registered backends are correctly seen

Failure to prepare the storage during backend or auth mechanism
manipulation will throw an InsufficientDataForMeaningfulAnswerException,
which is propagated to StorageNotAvailableException in the filesystem
layer via the FailedStorage helper class.

When a storage is unavailable not due to failure, but due to
insufficient data being available, a special 'indeterminate' status is
returned to the configuration UI.

Loading custom JS on a per-backend basis added needless complexity and
made dealing with async required. Now all backends/auth mechanisms load
custom JS in PHP

Introduces the basic password authentication mechanism, along with a
mechanism based on ownCloud credentials stored in the user session.

Change to lib/private is an extension of PermissionsMask, as
isSharable() override was missing.

Session credentials auth mechanism now disables sharing on applied
storages, as credentials will not be available.

…ing"

This reverts commit 75a5e6e.

The Rackspace/OpenStack differences have been split into separate auth
mechanisms, with correct legacy migration

SMB_OC has been merged with SMB, via the identifier aliases mechanism.
Legacy migration is done to the Session Credentials password mechanism

The SFTP backend now supports public key authentication alongside
password authentication.

phpseclib uses a default RSA key size of 1024, which will soon be
insufficient for good security. This boosts the generated key size to
2048-bit.