[BE] Denormalize community_id into post_aggregates for a 1000x speed-up when loading posts #3653

Credit to @phiresky for this idea, originally posted in comments of #2994

This PR adds community_id to post_aggregates (& a new index on post_aggregates) to enable joining community directly to post_aggregates when querying posts.

On lemm.ee, this optimization speeds up the query for front page of subscribed posts ~1000x, from several seconds to to just milliseconds. You can check a before/after of query plans here: https://gist.github.com/sunaurus/856e03165bb0c0010505afeebde45230