Feature Request: Updating blog's Profile ID triggers internal follower migration

### What

the web-finger for the last previous profile id/Handle should be preserved, to forward on users to the new profile ID/handle with a 'move' object

### Why

Changing the blog profile ID/handle creates a hard break that means followers of the old id no longer see updates.

### How

Suggested fix:
the plugin can simulate migrating a profile internally.
the webfinger for the last old blog profile ID/handle remains, but is set to forward/migrate followers on to the new/current profile id. it would probably also need one attached object with the 'Move' activity

BDD
GIVEN my current blog handle is @old@blog.address
WHEN i go to the activitypub plugin settings
AND change the profile ID to @new@blog.address
THEN the webfinger url 'blog.address/@old' still remains, but is now set to migrate/forward followers to new handle.

Start from the ActivityPub plugin settings flow where the profile ID changes, then trace how the webfinger URL for the profile handle is generated. Verify the old handle remains available and forwards followers to the new handle with the specified Move activity, covering the BDD scenario for @old@blog.address and @new@blog.address.

Written by the indexing model from the issue text.