Move tag fields beneath main content

Tags don’t change too often, so the fields, which are more complex than
simple text inputs, shouldn’t obscure the main content ones.

* Move tags into a partial
* Put partial at the bottom of each form
* Add a tags header and divider to separate fields from rest of form
Cleanup spacing of tag fields

* Use a smaller grid width to match earlier form fields
* Nest each item in a form-group for vertical spacing
* Re-word the section explanation
Avoid poltergeist overlay problems

Now the smart answer buttons are not at the bottom of the page, the
fixed workflow buttons obscure them when automating a click.

* Use trigger instead of click_on to always trigger the click on the
right element and avoid poltergeist’s co-ordinate warning
Merge pull request #271 from alphagov/tweak-tagging-of-collections

Tweak layout of tags on editions