About pages for topical events

Topical events often need some further explanation/supporting information. That content has previously been written in a featured news article, which is an abuse of the data model.

This branch adds support for "About pages", which can be associated with a topical event. About pages can be created in a tab in the admin UI, when viewing a topical event (the admin UI for topical events has also been reorganised/improved accordingly).

Visitors can access the events' about pages by clicking links that are added beneath the body copy on each topical event page.

The story in tracker mentions that users should be able to attach images to the About pages. That work is being prioritised separately, so that the work that's been done to date can be used sooner.

https://www.pivotaltracker.com/s/projects/367813/stories/45778979

@edds - There are some new bits of CSS in here that I'd hoped to discuss with you. We think it'd be worth you casting an eye over the front end changes, in case what I've done doesn't fit with your preferred approach to CSS re-use. I'll add a couple of specific line-comments to highlight a couple of specific things to you too…