autocomplete: Add "recent DM conversations" criterion

In @-mention autocomplete, users are suggested based on:
  1. Recent DM conversations.

Fixes part of zulip#228

Author: sm-sayedi

lib/model/autocomplete.dart

@@ -267,13 +267,34 @@ class MentionAutocompleteView extends ChangeNotifier {
 
   List<User>? _sortedUsers;
 
-  List<User> sortByRelevance({required List<User> users}) {
+  int compareByRelevance({
+    required User userA,
+    required User userB,
+  }) {
+    final dmPrecedence = store.recentDmConversationsView.compareByDms(userA, userB);
+    return dmPrecedence;
+  }
+
+  List<User> sortByRelevance({
+    required List<User> users,
+    required Narrow narrow,
+  }) {
+    switch (narrow) {
+      case StreamNarrow():
+      case TopicNarrow():
+      case DmNarrow():
+        users.sort((userA, userB) => compareByRelevance(
+          userA: userA,
+          userB: userB));
+      case AllMessagesNarrow():
+        // do nothing in this case for now
+    }
     return users;
   }
 
   void _sortUsers() {
     final users = store.users.values.toList();
-    _sortedUsers = sortByRelevance(users: users);
+    _sortedUsers = sortByRelevance(users: users, narrow: narrow);
   }
 
   Future<List<MentionAutocompleteResult>?> _computeResults(MentionAutocompleteQuery query) async {