diaspora should send AccountMigration message back when a federation message received for a moved account

I use Senya's Patch for this
The extra check is for satisfying tests, which don't create real database objects.

fixes #7902
closes #8288

When public fetch is requested with Poll guid, return parent
StatusMessage for it, which includes the Poll in its turn.

closes #7815

We only return `nil` when we know the person, but don't know the key or
the key is invalid, so it doesn't make sense to retry in this case. When
the person isn't known and can't be fetched we raise a DiscoveryError
which will be retried.

Also the errors were moved to the `Signable` module in the last release.

closes #7717

Related to diaspora/diaspora_federation#39

closes #7223

closes #7158

closes #7145

this happens if a remote person changed the diaspora-id manually (renamed
domain or something) in their database.

and do some refactorings

These are some initial tests, more to come.

It tests some features of Request, StatusMessage, Comment, Like,
Participation, Retraction, SignedRetraction, RelayableRetraction entities
receive process.