MPDX-178 Dropdown of previously created tags in all places you add a tag.

Added an auto-completing tag input dropdown interface for the contact tags list and task tags list.

For the tag list styling it uses the bootstrap-tagsinput library (https://github.com/timschlechter/bootstrap-tagsinput), though it's a bit of a misnomer as it doesn't actually require Twitter bootstrap itself and at this point, Bootstrap itself is not part of the MPDX code and I didn't add it here. For the auto-complete it uses the Typeahead.js library (https://github.com/twitter/typeahead.js).

I didn't add any tests for this, but it seems like a pretty front-end heavy feature that would be harder to test automatically.