Add comment validation rules.

* rating_check: Rating must be between 0 and 5
* duplicate_comment_check: Comment may not be a duplicate

Note that duplicate_comment_check is inefficient as it works the same
way as the logic for $already_rated (foreach loop over all user comments).
Ensure that a user's second comment rating is zero.
Only enable the spamcheck service if $params['api_key'] has something in it.

The config.php.dist file sets $config['akismet_key']  = '', which passes
the isset() test, but then isCommentAcceptable() will always return
false.
Merge branch 'comment-validation'
Merge branch 'disable-spamcheck-in-dev'