Monitor users' nicknames, send out a 'user changed' webhook when it
changes, and include the nickname in the webhook payload.
This also adds an `old_nickname` field to the 'user changed' webhook
payload. This is necessary for the Gitea user renaming logic. Since
Blender ID is more lenient with allowed characters, the username on
Gitea and the nickname on Blender ID will not exactly match. To detect
whether a rename of the Gitea user is actually required, the webhook
needs to know whether the Blender ID nickname actually changed or not.
