Add talk comment validation rules

Prevent duplicate comments and ratings outside of 0-5.
Also ensure that a user's second comment to a thread is always rated as zero.

JOINDIN-515 #close fixed.
No. The average talk rating excludes comments with a rating of zero. See the [get_talk_rating](https://github.com/joindin/joindin-api/blob/master/db/patch32.sql) SQL function.