Fix tag autocompletion features

* For reasons I haven't time to fathom, the behaviour of select2
  and/or Poltergeist and/or Capybara has changed with respect
  to the filling in of tags. Rather than identify which updated gem
  has caused this behaviour change, I've opted instead to stop using
  the rather "backdoor" behaviour of fiddling with select2's internals
  to set the value via JS, and instead talk to the input/click directly
  when autocompleting.