This commit removes the user() method from all PermissionBackend
subclasses. This method just encourages callers to pass around a
PermissionBackend when they really want to pass around a CurrentUser
and/or ChangeNotes.
Passing around heavy objects that represent a subsystem for the benefit
of getting smaller objects from it is an anti-pattern. We had this issue
at a larger scale with {Ref,Project,Change}Control that were often just
passed around to have a user and/or entity information. Removing the
user() methods should prevent that.
This refactoring also uncovered some wrongdoing that we already had in
the code as a direct result from passing PermissionBackends around (see
previous commit).
Removing user() comes at the (small) const that every PermissionBackend
has to implement #testCond by itsself, which seems fair and like a
better pattern in general.
Change-Id: Ibae1fc25e51228dca2acaf0faec76beee539da01
b682dae920