Add new interface for managing section and specialist sector tags

Relates to https://www.pivotaltracker.com/story/show/66501256.

**This pull request can't be merged until [this govuk_content_models change](https://github.com/alphagov/govuk_content_models/pull/149) is merged and the gem has bumped here.**

This adds new functionality to Panopticon to allow users to create and edit tags. This replaces the existing 'browse sections' feature with a new interface that includes both section tags and specialist sector tags, with potential to easily include other types of tags in the future.

Tags can be created at the top-level, or as children of existing tags. A tag requires a title and a slug (tag ID), and a description can be optionally added. An existing tag can have its title and description edited, and a curated list can be configured for section tags.

Users must have the 'manage_tags' permission to be able to use this feature.

![new-child-tag](https://cloud.githubusercontent.com/assets/71922/2592856/b9bac3fe-ba79-11e3-82e6-312d897523ee.png)
![edit-tag](https://cloud.githubusercontent.com/assets/71922/2592855/b9b9d4ee-ba79-11e3-8de2-04c5cafbf40a.png)
![new-tag](https://cloud.githubusercontent.com/assets/71922/2592858/b9bc3770-ba79-11e3-889b-134ab79e851a.png)
![tags-index](https://cloud.githubusercontent.com/assets/71922/2592857/b9bb4c98-ba79-11e3-8e7d-389a181b6c12.png)