Fix signin and initial loading with Tusk

Tusk is a fediverse Client for iOS currently in beta. These changes seem to be necessary for allowing Tusk to sign into pleroma instances.

• Tusk tries to load the user’s relationships after signin. The Mastodon relationship API has more attributes than we currently provide, and the missing attributes are marked as non-optional in the documentation. Since I don’t think we support these attributes, return default values for them.

• Tusk also tries to load the user’s filters after signin. Since we don’t support those either, this changeset fakes the endpoint for loading the list and always returns the empty list.

I’ll appreciate any kind of feedback.

TODO:

• Maybe also fake the other filter endpoints?