Feedback link on article pages

* Add API endpoint to fetch amount of entries for a given page/filter
* Code for above functionality was in use on special page already; moved
  that code to utils so both can access this and avoid code duplication
* add Javascript ajax call to fetch the amount of feedback and display a
  link on article page
* Update talk-page link to
  * If AFT is enabled: display link if there is *any* feedback
  * If AFT is disabled: only display link if there still is featured feedback

Bug: 42057
Change-Id: I03c815d8257aaa590ef639387c360d8356cd0d96
Merge "Feedback link on article pages"