Allow using same policy across several of admin/logged-in/frontend (and also inheriting?)

[16patsle]

Sometimes you don't need a separate policy for logged-in users, and sometimes you only need one in general. You might also want to override a single directive for logged-in users, while inheriting the rest from frontend. We should implement some toggles that allows the user to configure such behavior.

Something like:

• Option to "use this policy instead" (maybe using select element, but check for cyclic references)
• Option to "inherit unspecificed directives from this policy" (maybe tied to the above with a checkbox, or maybe a separate select)

Would need to figure out how there relate to each other and how they work together.