Student pending

not done yet... but I'm up for comments/criticism!

Basically, a student asks to join a group, and they're put on a list of pending students (or coaches), and then anyone in the group can decide whether or not to accept them. To do this, I added another field to memberships called 'pending', which is true or false. However, a lot of the methods in the app had to be changed to account for this, because before most methods just checked if a membership was present, (such as in `in_group`), but now everything has to check to see if memberships are pending or not. 

So far accepting a student/coach into a group doesn't work, but declining does. Mwahahahahaaaaa.

I was thinking that this couldn't be merged until some sort of notification system was in place, but @PragTob  thinks this isn't necessary. 

Still to do:
* [ ] alert group members that someone is asking to join their group somehow
* [ ] accept people wanting to join the group
* [ ] track down the probably many haml errors that happened during my rebase from master
* [ ] all da tests..... (this will most likely be a crazy fail right now)
* [ ] other stuff? Can a gif be fit in anywhere maybe?