Signing up to find pairs, coaches etc.

This 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)
* Add the ability to pass a `redirect_to` param to have the user redirected back to the current page
* Change `users#index` to only list their name, interests and location
* Make `users#index` the new homepage