feat: allow users to change their names

[wescopeland]

This PR gives users the ability to request name changes.

How it works

A new form has been added to /settings:

When the user attempts to submit the form, they get a confirm with this warning:

If the user confirms, the request is submitted and lives in the user_usernames table:

(note the new approved_at column)

In the management app, a new tool has been added:

The (1) badge indicates there is a pending username change request.

As an admin or moderator, I can view the list of pending username change requests:

If I click approve, the user's display_name value will be changed, something will be recorded to their account's audit log, and an email will be fired off:

This is the happy path case.

Alternatively, after 30 days, the username change request will "expire". It will no longer appear in the list of active requests, and the user will be able to request a new username change.

Other notes:

• After setting a display_name, users do have the ability to request reverting back to their original username (they can bypass this uniqueness check).
• Muted users cannot change their display_name.
• After a display_name change is approved, users are put on a 30 day cooldown before they can request another change.

[luchaos]

Random thought: does it auto-approve and/or allow direct edits for case changes? given that we act case-insensitive it should probably be possible to change case at will; Luchaos is the same as luchaos internally. Just bringing this up as we had case change requests in the past.
Please ignore if this particular case (no pun intended) has been addressed otherwise.
Wonderful work as usual, guys. It's a real treat following your conversations ❤️

[wescopeland]

Good question! It currently doesn't auto-approve those cases. But I think it would make a lot of sense to hunt that down in a subsequent PR before this goes live.