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 
@steventlamb should I refactor this in a cleanup pull?