Tidy up user policy a little
Tidy up user policy a little

Introduce `allow_self_only` to make it clearer when an action can only
be performed by the user to itself.  Remove redundant definition of
event_logs; it's covered by the existing implementation of `edit?`. Use
alias_method to link other permissions together where they are related
and share a permissions scheme.

alphagov/signonotron2,alphagov/signonotron2,alphagov/signonotron2,alphagov/signonotron2