Allow comments to be turned off for specific pages

[nacin]

Comments can occur on changesets, attachments, and files. They can also be inline, or made against the page generally. There should be some trac.ini configuration options to turn each of these six possibilities off.

For example, on core.trac, we would only like inline comments against attachments. We don't want general comments as those belong on the ticket. But we would probably want both inline and regular comments on commits if we enabled them there (which we won't do at first), and we wouldn't want comments on files at all.