Allow actor username to be changed

[angusmcleod]

@pmusaraj Next after https://github.com/discourse/discourse-activity-pub/pull/93 and https://github.com/discourse/discourse-activity-pub/pull/91.

The following will happen on remote services if you change the username:

• Mastodon: The username displayed for the actor will not change until someone performs a lookup of the new handle. Following / Posting etc will continue to work regardless of the username displayed.

• Discourse: The username will be updated the next time there is any interaction with the actor, including:

  • any activities are received from the actor; and
  • the actor is the subject of a lookup in the "Follow" modal.

See further: https://socialhub.activitypub.rocks/t/changeable-usernames/830/13?u=angus