This will allow us to write the build process in a full .NET language
(specifically, F# using the https://fake.build/ tool) rather than being
constrained by the limitations of MSBuild's XML format.

This converts from NuGet packages.config files scattered all over the
project to a single paket.dependencies file at the root, and
paket.references files on a per-directory basis. Advantages:

- Single source of truth for package version restrictions
- Paket can work out version numbers in project references, allowing
  easier upgrades to package dependencies
- Paket file format is plain text, not XML. Easier to read & modify

Also add required packages for https://fake.build/

GitVersionTask.alt is outdated (4.0.2-alpha) and no longer needed since
the official GitVersionTask package is on version 5.3.3 by now.

Also create a FKAE (https://fake.build/) script for calling
GitVersionTool and building the package version number used in the
Jenkins build.

The .NET Core SDK doesn't know to look for Mono assemblies on Linux, so
a FrameworkPathOverride is needed to make sure it finds them.

A recent change to FlexBridge includes using ###.### format for branch
numbers in Mercurial (e.g., 7500002.7000072 instead of 7000072). This is
causing some test failures in our SynchronizeAction tests, which don't
handle this change correctly. Until the tests can be brought up-to-date
with the latest changes in FlexBridge, we can just use a pinned version
from before that change was introduced.