Revert "Revert "Add bootstrap accessibility plugin. Fixes #710""

This reverts commit e70b7f9c3b2b610020bbdc41849200d70cf0eefb.

Conflicts:
	app/assets/javascripts/application.js
Disable bootstrap-accessibility dropdown extension due to conflict

After much agonising I think this is the simplest solution for now. We
get all the accessibility benefits of bootstrap-accessibility (minus
the things on dropdowns) and the usability benefits of
bootstrap-select.