This adds a concept of `default_actor` that `HasWiki` instances must
implement. This is implemented differently for projects and groups.
The contract is that `default_actor` must return a User with ownership
privileges over the object.

Wiki Owner differs from Owner of these models as wiki_owner must have
the type `User`, but `Project#owner` can be a `Group` for instance.
Groups can have many owners, so we select the first one, before
ascending the chain of parent groups.

This is vital since otherwise `HasWiki#wiki` will return wikis that
cannot save their own pages.