Add "x spots left" banner to open events on events page and home page

[ddgriffin]

We'd like to add a banner to make it easy for people to see which events are still open for applications/signups. (see screenshot)

The frontend goes in:

1. _event.html.haml & _event_home.html.haml under views > events, and of course the CSS (custom.css.sass)

• you'll want to go in there and add some logic to check for state: accepted (it might already have this logic, actually), and on the ones that are in state: accepted, have it display the banner by pulling the number of spots left (you can find the logic/math to get the number because it's already displayed on events)

[cheyenneweaver]

[cheyenneweaver]

@tmctigrit this looks awesome!!! Lets merge it with master :-)