Add a js date picker back to add-bagel-page.

Removal of prototype broke the old date picker. Use the jquery plugin,
pickadate as the replacement.

Enable asset pipeline and move all assets to it.
Remove unused gems.
Remove unused autotest.
Re-add player name autocomplete when adding a bagel.

The previous implementation used an old library.

The new implmentation uses jquery exclusively.
Switch to scss fileformat.

So formatting from css is easier.
Make the new-player form work again.
Useless "back" link.