Finish up post count support in tags API

Refs #4521
- Handle 'include' query param in tags API.
- Add post_count support when fetching a tag with findOne.
- Remove post_count from options.include after processing.
- Extra database query no longer used to fetch post_count.
Merge pull request #4686 from jaswilli/tag-api-post-count

Finish up post count support in tags API