FEATURE: Allow actor preferred username changed

[angusmcleod]

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

[pmusaraj]

Looks good, will merge, we can address the username change period question in a followup, if it's an issue.

[pmusaraj]

@angusmcleod should the plugin also account for username change period in core? Not sure if that raises an error, and note that the setting also blocks username changes entirely if set to 0.

[angusmcleod]

The username changes handled by the ActorHandler will be unaffected by username_change_period, as changes are not filtered through the UserGaurdian that applies it. Even though we leverage some User username logic for the Actor username, e.g. UsernameValidator, I don't think it'd make sense to apply the UserGuardian, or restrictions like username_change_period to Actor changes, as different considerations are in play. We may restrict Actor username changes, but it will be for different reasons, e.g. that a plurality of other ActivityPub services do not support changes beyond a certain period. To put it another way, the considerations pertinent to Actors are sometimes overlapping, but mostly different from the considerations pertinent to Users (or other models), so I think it's best to keep them separate (which is also why we have a separate model for them).