Add date logic to update language when an Event passed

[courte]

Description
Events come, events go, and we never talk about them the same way when they're gone... except on the CodeMontage Platform. Right now on the event show page ("Join us"), on the events index (all the events are listed together), and perhaps in other places on the site - there is no distinction made for past events. It would be great if we could dynamically change event-related language if an event has already passed.

Ask
Change the language on the events show & index templates according to whether an event has passed or not.

To Do

• Differentiate between upcoming and past events in the Event model
• Change language in events show template button and text from "RSVP" and "Join Us"
• Change listing in events index template to include a "Past Events" heading

Additional Info

[courte]

Resolved with #348.