Feature/remove activities controller

While testing notification rules, I noticed that there was still an `ActivitiesController` in place that I had been using while I put `PublicActivity` into place. Since the UI for this isn't really well put together (or necessary), I think it's best that this controller gets removed altogether.

I started off completely removing the controller, but then realised that there are a few things still being used -  styles for the activity listing from `app/assets/stylesheets/activities.css.scss`, and views from `app/views/activities/*`. So, I've just removed the controller and controller spec, while leaving everything else in place.

All specs passing last time I checked, should be good to merge.