Out: Roll-up the total communities for each instance for display on the instance view

[tgxn]

allow filtering for " hide instances with no communities"

[tgxn]

This is done as a overall "count" in the meta file, and filtering the main community list based on this. https://lemmyverse.net/instance/techy.news/communities

I'll probably put the overall count in the instances files too, at some point.