I couldn't completely remove the global room in one commit, but this
solves basically every problem with it by making it no longer a `Room`.
In particular, this means:
- It's no longer of type `Room`
- It's no longer in the `Rooms.rooms` table
- Its class name is now `GlobalRoomState` rather than `GlobalRoom`
- It no longer tracks its own user list (online user count is now
provided by `Users.onlineCount`)
- It's no longer a socket channel (there's new syntax for "send this
message to every user")
Not having prefer-const on the JS side makes JS -> TS refactors really
unreadable. This commit just auto-fixes it so we're using
`prefer-const` everywhere.