Showing owner/author and last-updated date in activity index.

[#46483045]
Supports tracking the last user to update an activity.
Gets rid of "should" in interactive_page_spec
Got an extra column in there somehow
Better rspec idiom in activity model spec
Pages now know when they or their embeddables or interactives were last updated.
Activities can show updated_at for themselves and all descendants

For [#46483045] - now the "last updated" can change if a page or embeddable is edited but the activity object is untouched.