Replace tagsinput.js with tagsmanager.js

this js library is simpler and does what we
want it to. the markup it generates for tags
has a separate span for the need id which we
can use to bind click events to. tagsinput
was putting the remove link and tag content
in the same span.
Making changes to work with different JS library
Using updated version of govuk_content_models

this has nil checks for need_ids so artefacts
without need_ids don't error during validations.