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.
Embeddables try to know their activity
Changes should update who made the change on index page
We don't really need deep_updated_at

Because most changes to descendant objects will change the activity when they change changed_by.
We don't need the deep_updated_at stuff with changed_by being updated
Whitespace/comment cleanup
Using let in lightweight_activity_controller_spec
More test updates

* Using Poltergeist to do JS testing with PhantomJS instead of whatever Capybara uses by default, so Guard isn't so annoying
* More use of factories and let to clean up test code
Testing "who changed activity" - works best when the activity is valid.