feat: add option to change username

[Fijxu]

Tested and it works fine. I made this for people that want to change their username on public Invidious instances without having to create a new account or having to contact the instance maintainer to change their username. It also updates the materialized views sha256 ids, so if a user changed their account name from user to user2 and a new user registers on the instance with the username user, there shouldn't be a relation collision (when the collision happens, this is displayed: relation "subscriptions_sha256" already exists)

Tested:

• Changing username from user1 to user2, subscriptions are maintained across username changes
• Playlist author (for Invidious playlists) updated on username change.
• Changing the username from user2 back to user1 works well.
• If the user tries to change their username to one that already exists on the database, a "Username is already taken, use another one" will be returned. (Although someone can still register or change their username with an already taken username but with a slight unnoticeable SPACE character on it, it's possible to have a user called user1 and user1 )

[Fijxu]

Note: This conflicts with #2469 due to the use of materialized views.

[SamantazFox]

We'd need to use UUIDs of some sort to differenciate accounts at some point. I'm worried that account name switching could be used to find if someone else is registered on this instance.

[Fijxu]

I'm worried that account name switching could be used to find if someone else is registered on this instance.

That is already possible in the login page, if you insert a registered user with a wrong password it will return Wrong username or password.

Unless you are referring to another case I'm missing