Problem: Mainline 4.0 servers will request hashtag feeds as well as user feeds now.

[politas]

Mainline 4.0.x introduced a feature to allow users to follow hashtags, involving servers requesting not just particular users' feeds, but feeds of specified hashtags. Ecko users will be less represented in hashtag based discussions if Ecko does not respond appropriately to those requests.

We need to implement the API changes found in mastodon/mastodon#18809 that allow servers to pull all posts to given hashtags.

See also #454 request to implement the ability for users to follow hashtags and to pull said feeds from other servers.