Support JSON requests to create, update and publish tags

https://www.pivotaltracker.com/story/show/79177806

Soon, the canonical source of section and specialist sector/topic tags will be [Collections Publisher](https://github.com/alphagov/panopticon), which will update both Panopticon and Content Store when changes are made. At this point, we'll be removing the user interface to manage tags from Panopticon completely.

To support these changes, we need to change Panopticon to support JSON requests to change tags. We already have controllers and actions which do the things we need, but until now they've only supported the HTML format. 

These changes add proper support for JSON requests, and add extra test coverage for the expected behaviour. This also removes the legacy curated list functionality, which has not been used since the mainstream browse redesign in July, and changes the resourceful routes for tags so that they include the `tag_type` and `tag_id` attributes.