Conversation System: Add support for comments reporting

[vladolaru]

Add the option to report a certain comment. Readers could flag a certain comment with a required short message about why they reported the said comment. This could include false information, bad language, errors, etc.

Basically, it's a tool to assist with moderation. The link should reside in the "More" dropdown and use AJAX to send. Only registered users should be allowed to report(?)

The Correspondent has a very nice take on this in the form of a "letter" of sorts to the article author:

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Note: Related to #268