Required checkboxes

Sorry, this is kind of a big one. Turned out a half dozen other things had to be done before "requiring checkboxes" (as the branch name suggests) could be done.

The app now:
- Requires you to check exactly 3 skills/trait checkboxes to advance. (The 3 is variable, we can change it to whatever we need.)
- Uses a filter for determining which skills/traits to display to the user
- Persists skill choices made on skills page to the write your pitch page
- Has a new controller, write-pitch-controller, to keep things better organized and easier to understand
- pitch-controller has been renamed pitch-main-controller to match its view name
- removed/cleaned up some commented junk and experiments