Add backend method to retrieve all exercises from database

Summary:
Stylistical changes to bootstrap carousel controls

Make exercise summary a carousel with whole table on one slide and data drill down on the other

Test Plan:
/data/exercise-summary/all should have roughly 400 results as of date of commit

Controls should have fixed position on the page

Nice sliding effect of two pages

Reviewers: jace, chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D2240