When we first deploy temporary account autocreation, we want temporary
users to resemble anon users more than registered users, for minimal
community impact.
If we add them to the 'user' group, then they will automatically get
whatever rights a wiki currently assigns to registered users. If we
don't, they will only get rights that are currently given to everyone
(same as anons).
Therefore stop adding them to the 'user' group, and instead add them
to a new 'temp' group, in case site admins want to give them extra
rights.
Bug: T340457
Depends-On: I6333fa2289f3142d1b5cedf2cc910ced3ba5019e
Change-Id: Idb46252fe6533ab0a5410bdb3093043cb359c206
dd2f898f86