Add a "List" or "Multicommunity" feed

[JackFromWisconsin]

Hello,
I would like a feature to create "lists" of communities to throw into a single feed. Basically, a way to view !science, !fuckcars, and !socialism (for example) in a feed.

I am torn on whether this should be private and defined per user, or alternatively to allow multicommunities to be public. Private seems easier and less work for moderators so that's probably best.

Kind of a low-priority but would greatly enhance my Lemmy experience.

[Kap2022]

I think this would be useful to help an instance consolidate a variety of feeds on a specific topic that are spread across multiple instances.

[dessalines]

Dupe of #818