Bootstrap-based UI

Initial rewrite of the Azkaban UI to be based on Bootstrap 3. All of the main webapp pages are mostly done. JMX page and a few pages that seem to be unreachable from main webapp still need to be re-styled. Motivation:

1. Azkaban CSS is due for clean-up as it contains the last remaining pieces of Azkaban 1.
2. Standardizing UI elements on Bootstrap make it much easier to add more UI features.
3. Bootstrap 3 is easy to extend and re-skin.