Style content lists

Before:

![screen shot 2014-12-08 at 14 48 58](https://cloud.githubusercontent.com/assets/68009/5340884/5ecb94e8-7ee9-11e4-9ead-38b348a62f97.png)

After:
![screen shot 2014-12-08 at 14 49 32](https://cloud.githubusercontent.com/assets/68009/5340900/765dabfa-7ee9-11e4-8224-1a409adfc69a.png)

This pull request:
- Improves the visual styling of the lists of content
- Uses more semantically appropriate tags for the content (lists instead of h2s)
- Removes ruby that checks if the section is collapsible as all of these sections are no longer collapsible

Trello ticket: https://trello.com/c/Z2z8wKgh/432-hmrc-manuals-frontend-3