Add super user privilege

[richardscull]

This PR introduces new UserPrivilege of SuperUser, this privilege can be obtained through hidden !claimowner command. For the security, the command requires two things to be true:

• User should provide a secret key, which would be printed in server console on startup if no other SuperUser exists.
• No other users should have SuperUser privilege.

Since SuperUser is one of the highest roles, it can't be given to anyone else using API or Bancho commands, so if you want to transfer SuperUser to another account, add DEMOTE_SUPERUSER_ON_STARTUP_USE_THIS_IF_SOMEONE_STOLEN_YOUR_SUPERUSER_ACCOUNT=true in the .env for your environment at startup. It will demote previous user who had SuperUser and print new secret key in console.

Also, all Developer privileged commands/actions now require SuperUser instead. To this, only SuperUser will not be kicked from the server if the maintenance mode will be enabled.

---

This PR should help self-hosters who don't have direct DB access or don't have the knowledge to get initial privileges for the superuser account.