DM guessing: prefer oldest joined member

In the DM guessing code, prefer the oldest joined member if there's
anyone in the rom other than us. Otherwise, fall back to the old
behaviour.

Fixes https://github.com/vector-im/riot-web/issues/4288

src/Rooms.js

@@ -144,7 +144,18 @@ export function guessDMRoomTarget(room, me) {
     let oldestTs;
     let oldestUser;
 
-    // Pick the user who's been here longest (and isn't us)
+    // Pick the joined user who's been here longest (and isn't us),
+    for (const user of room.getJoinedMembers()) {
+        if (user.userId == me.userId) continue;
+
+        if (oldestTs === undefined || user.events.member.getTs() < oldestTs) {
+            oldestUser = user;
+            oldestTs = user.events.member.getTs();
+        }
+    }
+    if (oldestUser) return oldestUser;
+
+    // if there are no joined members other than us, use the oldest member
     for (const user of room.currentState.getMembers()) {
         if (user.userId == me.userId) continue;