Show commenter approved count in moderation screens

[tellyworth]

It would be useful to display the number of previously approved comments for each comment author somewhere on the comment screen.

That number could also be sent in the comment-check API call.

There are currently no indexes on the user_id or comment_author_email columns, so this might be too slow to work on large blogs, but let's try a patch now and see how it works. If it's too slow we can hold it back.

Related core ticket:

​http://plugins.trac.wordpress.org/ticket/1186

[tellyworth]

Oops, wrong URL above. This is the related core ticket:

​http://core.trac.wordpress.org/ticket/14711

[tellyworth]

Partial solution in [294390]. It's enabled only when the comment_moderation or comment_whitelist option is set.

Some styling fixes are still required - the gap below is unintentional.

[tellyworth]

Option and filter added in [297212]