`filterPostElements` util: Communities include/exclude option

[marcustyphoon]

Concept

Perhaps we should add an option to filterPostElements that—possibly by default, following the includeFiltered precedent—includes or does not include all "communities" posts, and do a pass confirming that none of our scripts affect them until we explicitly want to enable them doing so and have handled the API differences those posts have.

[marcustyphoon]

Probably easier to address after #1482 or equivalent.