Tweak layout of tags on editions

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

![screen shot 2014-10-23 at 14 35 00](https://cloud.githubusercontent.com/assets/319055/4753750/86054418-5ab9-11e4-8691-f0ab2ba07202.png)