Switch to Either in loginByRememberToken return type

Most login related functions were already using Either for returning
login related errors.  Convert loginByRememberToken from Maybe to
Either to be consistent with the rest of this module's error handling.

Fix unit tests for the new loginByRememberToken type
Return () instead of AuthUser from forceLogin

forceLogin was returning an AuthUser for no apparent reason.  It
wasn't clear what the callee of this function would do with the
returned (unchanged) AuthUser, so use ().
Major version bump
Merge remote-tracking branch 'origin/0.11'

Conflicts:
	snap.cabal