Make events on group detail page clickable

This follows the design from the wireframes. Group admins see a dropdown menu and everyone else sees a right-facing chevron. For all users, tapping on the event row navigates to the event detail page.

(Note: This reimplements 9a9d77b1 and I plan to rebase this pull once that commit has been merged into develop)

Fixes #191 
It's strange that the base template only knows about a var that is only used by a downstream template. I think it would make more sense to make a block for the `class` and implement this switch in the template that extends this. Slightly less dry, but better encapsulation. Then the comment can come out because it will be easier to gleen from the context.