Allows current mentors to checkin attendees
doc improvement
Links directly to checkin page if not admin
Moves event details to partial
Better "event has passed" warning handling

Only shows the warning if more than a month old, based on a new
“far_past?” method on Date.
Includes Events of type "Event" in non_visits
Add button to Take Attendance
Merge pull request #42 from uwdreamproject/matt-develop

Event checkin improvements for non-admins