Support very large channel membership lists (#11939)

Fixes the channel membership dialogue for the zed channel by not
downloading all 111k people in one go.

Release Notes:

- N/A

crates/collab/src/tests/channel_tests.rs

@@ -99,7 +99,7 @@ async fn test_core_channels(
         .channel_store()
         .update(cx_a, |store, cx| {
             assert!(!store.has_pending_channel_invite(channel_a_id, client_b.user_id().unwrap()));
-            store.get_channel_member_details(channel_a_id, cx)
+            store.fuzzy_search_members(channel_a_id, "".to_string(), 10, cx)
         })
         .await
         .unwrap();