Comments moderations feature

[ksurya]

Broadly speaking, a auto-comment moderation mechanism is needed.

1. Automatically detect, block users commenting on submissions at very high rate. (i.e., we can put a limit to the number of comments user can submit)
2. Hide first few user's comments for admin-approval until they are found trusted commenters
3. Identify users who have been flagged more than once, and block them.