Create share by e-mail after invitation accepted

[mirekys]

Scenario

1. Let's assume we have user A who starts the invitation workflow by sending an invite link to user B's e-mail b@example.org he knows.
2. User B accepts the invite link by going through mesh directory and logging into his provider of choice
3. User A now wants to share something to B
4. At this moment, user A still knows only the user B's e-mail (b@example.org)

The problem

User A has currently no means of determining grantee.opaqueid used in the invitation sharing workflow, he doesn't even know which provider.idp was used by user B to accept the invite.

1. User A should be able to create a share for the user B by using the e-mail adress used in the invitation workflow:

$ cmd/reva/reva -insecure ocm-share-create -grantee b@example.org /home/test

or/and

2. User A should be able to list remote users that accepted the invites he sent. Such accepted invite records should contain some label, that helps user A to recognize, who actually accepted the invite using which invite link (the label would be the used e-mail address b@example.com in this case). User A could then find all necessary sharing details about user B (provider & opaqueid) in this listing based on the label he already knows and any additional user info exchanged at the moment the invite was accepted.

[labkode]

@mirekys can you start implementing option 2)? This is the foundation

[mirekys]

Implemented in #1527