appview/notify: use sets to manage participant list #874

I think map[T]struct{} is just fine, but I do see the values of this abstraction. recipients.Remove(actorDid) seems pretty cool tbh.

How do you think @anirudh.fi ?

there are a couple more usecases for sets that i can think of already across the codebase!

(this also fixes the bug where user who is a participant + mentioned would result in a double notification)

As a seasoned Go writer (I jest) who has never needed this abstraction before, I'm inclined towards a map[T]struct{} but if we're using this elsewhere, then no biggie. @oppi.li thoughts on just vendoring/in-tree'ing this code? Could have a subpackage somewhere…

yes, lets move it into the tree! although i don't expect the library to change much.