Don't throw JS errors if there's nothing to scroll

This was breaking on Travis - if there was no #end-scroll-track div, the scrolling-interactive script generated errors.
Comments and cleanups in ApplicationController
Tweaks to squeeze speed on OpenResponseAnswersController
Install lol_dba to look for indexes (no help)
Add and configure Bullet for database optimizing
Eager loading to make pages lighter on the DB
Updating Guardfile for recent refactorings
Caching expensive computed methods

(N.B. also catches a common exception we haven't figured out how to deal with yet).
Eager load answers with run