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
The Collections class seems to be responsible for creating collections and grouping them.
Would it be worth extracting a `mainstream_browse_page_tags_collection` and a `topic_tags_collection` object?
That would open `Collections` to be refactored into an `CollectionSorter` or `CollectionCatalog` that would just be responsible for the grouping.