Speed up comment list queries

[dessalines]

Unfortunately the comment table doesn't have community_id (only post_id), so it can't benefit from any of the fast same-table community id filtering. And it looks like the subscribed and moderator view fetches for comment are quite slow.

The only way around this IMO (without getting rid of comment views on the home and community pages entirely), is to add a redundant community_id column to the comment table.

I'll reference this in another issue.

Originally posted by @dessalines in #6396 (comment)

[Nutomic]

Sounds reasonable.