Extract singning of AccountMigration to a different module and introduce alternative form for account migration message

[cmrd-senya]

As discussed at https://github.com/diaspora/diaspora/issues/908, if we want to make AccountMigration object fetchable from the new pod and at the same time we don't want to keep private key of the old person on the new pod, we need to save signature of the old person.

In order to do that, we need two things:

• Allow AccountMigration model from diaspora* to compute a signature without instantiating the entity, so we can compute the signature and store it to the database at the time when we build the AccountMigration object at the new home pod (with old private key as user input).
• Allow alternative form of AccountMigration entity where author is the new user and old user is defined by a separate parameter.

This PR introduces these changes.

In this branch I develop changes in diaspora which make use of these changes, but only of the first part: it saves signature for AccountMigration to the DB. Also in that branch I resend AccountDeletion/AccountMigration to the sender if we received a message for a closed account. I'll prepare that branch as a PR soon.

[cmrd-senya]

Updated

[cmrd-senya]

Updated

[SuperTux88]

Thank you, merged 🍪 (I changed one little thing after merging it with the spec, that I didn't see yesterday, but didn't want to do another round for this)

[SuperTux88]

Oh, pushed it to the wrong repo, now it's merged ...