Get pending federation invitations from header (instead invitations endpoint)

blocked by #https://github.com/nextcloud/talk-android/issues/217

Pending invitations for federation can now be received from server in the "get rooms" header. The talk app still uses the invitations endpoint to get these infos.

Using these infos from get rooms header will work for one account, but with multiple accounts this approach won't be able to show a badge whenever there are pending invitations for other accounts.

Right now i request pending invitations for all accounts when the conversation list is shown. If there are pending invitations for other accounts, a badge (red dot) is shown on the account avatar.

So for the iOS app it may make sense to request rooms for all accounts (to store everything in database) and then have the pending invitations infos as well.
For the android app, requesting rooms for all accounts at once is not done yet as there is no offline support.
So for the android app it's maybe better to wait with this until we finally implement offline support.

Start by reading the dependency in issue #217, then inspect the Android app's current invitations-endpoint flow and the get rooms response header. Clarify whether offline support and requesting rooms for all accounts are prerequisites before defining the change; done should include pending-invitation badges for the relevant accounts without relying on the invitations endpoint.