Refactor ability.rb into Policies
## What does this MR do?
Factors out `ability.rb` into a new abstraction - the "policy" (stored in `app/policies`). A policy is a class named `#{class_name}Policy` (looked up automatically as needed) that implements `rules` as follows:
``` ruby
class ThingPolicy < BasePolicy
def rules
@user # this is a user to determine abilities for, optionally nil in the anonymous case
@subject # this is the subject of the ability, guaranteed to be an instance of `Thing`
can! :some_ability # grant the :some_ability permission
cannot! :some_ability # ensure that :some_ability is not allowed. this overrides any `can!` that is called before or after
delegate! @subject.other_thing # merge the abilities (can!) and prohibitions (cannot!) from `@subject.other_thing`
can? :some_ability # test whether, so far, :some_ability is allowed
end
def anonymous_rules
# optional. if not implemented `rules` is called where `@user` is nil. otherwise this method is called when `@user` is nil.
end
end
```
See merge request !5796
