Turn "Drupal comments" into "Private comments" #2558

#2558 (NOTE: the branch name does **not** match the issue number!)

* updates the names of comment_sources sources: standardizes capitalization and renames "Drupal" to "Private" comments.
* adds "smart" help text to link users to update app settings if they want to enable/disable/change comment source.
* updated "post comment" permissions to only allow site members
* remove "Log in to post comments" (comment_forbidden) link for anonymous users.