remove bootstrap

Remove our references to bootstrap. This was a cause of scrolling slowness in ace. Many selectors were matching within the ace dom, causing layout calculations to be slow. There are still several unqualified selectors we should look at specializing. Our reference for scrolling speed - of Ace in a Dart chrome app - could be Ra. It's fast.

- remove bootstrap references from css and the pubspecs
- in-lined and fixed UI regressions where we had been using bootstrap's css
- added comments where we should do more css selector investigation

@ussuri, @emilycodes 
This already only applies to scrollbars -- what else did you have in mind?