Allow tagging of collections to editions

PLEASE CONFIRM ALL TASKS BELOW ARE COMPLETE BEFORE MERGING.

https://www.pivotaltracker.com/story/show/79283660

This adds the ability to tag mainstream content to mainstream browse pages and (new-style) topics direct in the publisher rather than in Panopticon.

This is required to support both email alerts for topics and the publishing 2.0 model.

Fields are similar to those in Whitehall, and look like this:
![screenshot 2014-10-22 11 59 01](https://cloud.githubusercontent.com/assets/109225/4735624/6b120948-59e2-11e4-9de8-43bb8deebca7.png)

- [ ] Code review (anyone)
- [ ] Interface review (@fofr)
- [ ] Timing and notification agreed with Content
- [ ] Coordination with these tickets: https://www.pivotaltracker.com/story/show/79293166 and https://www.pivotaltracker.com/story/show/79283726
It comes down to how we see this extending: can we see this heading in the direction that we will need to add behaviour to the different collections (enough to warrant extraction into discrete objects)? If so, I do feel it's worthwhile. Otherwise, happy to leave as is and wait for more info.