Replace twitter-bootstrap-rails with bootstrap-sass

The bootstrap-sass gem[1] is the official port of Bootstrap to Sass,
which is what we are using. This commit moves us to that gem, which also
happens to bring us to a newer version of Bootstrap, so I made the
required modifications at the same time.

[1]: https://github.com/twbs/bootstrap-sass

Change-Id: I445046b63201ae8854ef3c0062c9262419dd6162