Updated meta helpers to use tag meta data if present

No issue

- Tag Meta title and description override default response
- Tag Meta title present on all pages
- Tag Meta description available only on first page
 - Updates tests
Run tag add operations in sequence

Closes #4697
- Run tag add operations in sequence instead of in parallel
  since generateSlug will hand out duplicate slugs until one of
  them is committed to the database.
- Add test.
- Misc cleanup of method parameter names and jshint hints.
Merge pull request #4693 from cobbspur/metatags

Updated meta helpers to use tag meta data if present
Merge pull request #4698 from jaswilli/tag-dup-slugs

Run tag add operations in sequence
Merge branch 'master' of https://github.com/TryGhost/Ghost