Style select2 tags/input to match existing styles

* Upgrade select2-bootstrap css to the latest Bootstrap 3 version from
https://github.com/fk/select2-bootstrap-css
* Include the select2 styles after bootstrap, but before our own styles
* Separate out key tag styles into silent SASS classes for extension by
select2
Remove visited tag styles

Purple visited tag styles aren’t needed yet, and cause breakages with
select2 tags
Align tag text input with existing tag choices
Fix failure to act when all common tags removed