[API] Return post count for each tag

closes #4521
- if '?include=post_count' present, query db for tag/post_count information
  and append it to returned JSON
- fixed linter errors
- added integration test
- parsing to int as knex (or PostGres driver) is returning bigint as strings
- iterating through a smaller collection to avoid exception
Merge pull request #4654 from ekulabuhov/4521_TagPostCount

[API] Return post count for each tag