squash all private comment commits #2558

* Renames "Drupal comments" to "Private comments"
* Only site members can add and view private comments
* Removes the "Log in to post comments" link
* Adds link to App Settings on node form comment settings
* Update behat tests
Merge pull request #2756 from openscholar/2258-priv-comments

squash all private comment commits #2558