Notification for new posts from selected communities

[VicMCA]

Pre-Flight checklist

• Did you check to see if this issue already exists?
• This is a single feature request. (Do not put multiple feature requests in one issue)
• This is not a question or discussion. (Use https://lemmy.ml/c/jerboa for that)

Describe The Feature Request Below

Would it be feasible to add communities to a "watch for new posts" list of sorts, so that we could receive notifications on Android/iOS for them? e.g.: I subscribed to the !news@lemmyrs.org and would like to be notified about new posts there.

Probably it'd be good to limit how often Jerboa could check for updates, with a minimum of 1h perhaps.

[dessalines]

Besides RSS, a bigger task for next year is to rework notifications for lemmy: LemmyNet/lemmy#2631

[dessalines]

Dupe of #275