Ability to rename homed users

I so no real technical reason why we can't rename homed users. It might invalidate a cache or two but that's fine

This would free us up to hide the very existence of a username from all of GNOME. Right now, usernames only really appear wherever we can create a new user. We expect the user to set this username, and say that they'll never be able to change it, which I am certain is confusing.

Instead, we should auto-generate a username from the real name. If they care about the username, they can override it (via something like homectl rename <autogenerated username> <chosen username>). We could even note if the username was auto-generator or not, and if it was we can change it whenever the real name is changed. In any way, adding this feature to homed would give lots of flexibility

AccountsService already supports this kind of thing... So I don't know why the UI says everywhere that it's not supported