Remove existing curated list code

The new stuff will work very differently, so let's start with a clean slate.
Add browse section index page
Restrict access to users with permission
Add basic edit form for browse sections (tags)
UI for editing the curated list for a section
Ignore blank artefact_ids

This means that dropdowns with nothing selected, and the hidden dropdown that
acts as a template won't be included, making for garbage data.
Highlight top-level sections
Remove Tag UI and API

The UI was useful for editing sections, the API was used by Frontend/Rummager
back in the day to render browse pages.
Add text to explain how the curated list is used
Merge pull request #61 from alphagov/browse_page_ui

Browse page UI