Version 3.0: AJAX pager, better anti-spam measures, etc.

If a page has more than 100 comments, pager is used instead of trying to
load all comments at once, which could cause OOM issues when a page has
lots and lots of comments.

Using external links in comments now requires the new 'commentlinks' user
right, which is given to the autoconfirmed group by default. Without this
user right, the user cannot post content that contains *any* external
links.

Posting "spam" (content that matches $wgSpamRegex, $wgSummarySpamRegex or
other such filters which are hooked to the Comments::isSpam hook) requires
the commentadmin user right.

Patch by MediaWiki.org user SimonLitt with some very minor cleanup by me.

Change-Id: I4d1d67c745999d31d000b15893e7af93dbd6d41c