Styled task tags using bootstrap-tagsinput

This is an initial step toward JIRA ticket MPDX-178, "Dropdown of previously created tags in all places you add a tag." I have pulled in the library, https://github.com/timschlechter/bootstrap-tagsinput and added the tagsinput styling to the task tags field.

Next steps toward resolving the ticket are to added the same styling to the contact tags field, and to make is so that it has an auto-complete dropdown as per the Typeahead example given in http://timschlechter.github.io/bootstrap-tagsinput/examples/