Signing up to find pairs, coaches etc.

Implements a first version of #63 

This pull request does:

* Require signin for submitting an application
* After signup redirect to `users#edit` and ask for completing their profile
* On `users#edit` add a list of interests users can check (like "finding a pair", "finding a coach", etc)
* Change `users#index` to only list their name, interests and location
* Make `users#index` the new homepage
* Add the ability to pass a `redirect_to` param to have the user redirected back to the current page
* Simplify the application form markup

I have deployed this on staging: http://rgsoc-teams-staging.herokuapp.com. We'd probably want to delete existing users from the 2014 teams app (should we have any, yet).

I think this can already replace [our spreadsheet from last year](https://docs.google.com/spreadsheet/ccc?key=0AjmbYjwdIlFZdHdMcERKVWxaVkVNX1BYRnlPVkV1bFE&usp=sharing)