Edit User always refers to "your username" even when editing another user

[caesarsgrunt]

When editing a user, it always says "Your username cannot be changed" next to the username field, even if it is not yourself that you are editing.

It should not mention you if you are not editing yourself. Instead it should say something like "This user's username cannot be changed", or just say "Username cannot be changed" for all users.

[nacin]

(In [13820]) Use a generic string instead of "Your username" in user-edit.php to account for non-profile edits. props caesarsgrunt, fixes #11353