Implement importing of 'Followers' list when moving actors

[dmitrizagidulin]

We have two export/import use cases:

1. Export from, and import to, the same actor. (For example, export from https://alice.example actor, delete the db, and then import back to the https://alice.example actor).

2. Export from one actor (e.g. https://provider.example/alice) and import to a DIFFERENT actor (e.g. move to https://alice2.example.com)

In the first case (same actor), just importing the Following collection back into the database is sufficient. We can assume that all the actors you're Following still have the 'Follow' requests recorded on their side, and so are sending you all their updates. (This may not always be the case, but close enough for now).

This issue is to implement the second case -- an export/import cycle where you're switching to a different actor.

• When importing the 'Followers' list from the old actor to a new actor, go through this list of accounts that are following you. For each account:
  • Add the account to the 'Followers' accounts table (if it does not exist already)
  • Send a Move activity to each follower (saying that you've moved from the old actor to the new). See https://codeberg.org/fediverse/fep/src/branch/main/fep/7628/fep-7628.md for details.