Merge content store and content API data.

Treats Content API as source of truth for existence of tags, for now.

Ticket: https://www.pivotaltracker.com/story/show/77966226

Merges work from https://www.pivotaltracker.com/story/show/77198458 and https://www.pivotaltracker.com/story/show/77197836 in order to provide simplified content for https://www.pivotaltracker.com/story/show/75806906