user [nfc]: Refer to "user store" rather than "store.users" in text

At the end of this series, there won't be a `users` member on the
store, so this code-like reference will no longer make sense.
The underlying meaning in each of these contexts will still be apt,
though: the data structure where the known users are tracked.  So
update them to make that reference more generically.

Author: gnprice

test/model/autocomplete_test.dart

@@ -369,7 +369,7 @@ void main() {
     }
   });
 
-  test('MentionAutocompleteView mutating store.users while in progress does not '
+  test('MentionAutocompleteView mutating user store while in progress does not '
       'prevent query from finishing', () async {
     const narrow = ChannelNarrow(1);
     final store = eg.store();

test/widgets/recent_dm_conversations_test.dart

@@ -215,7 +215,7 @@ void main() {
           checkTitle(tester, user.fullName);
         });
 
-        testWidgets('no error when user somehow missing from store.users', (tester) async {
+        testWidgets('no error when user somehow missing from user store', (tester) async {
           final user = eg.user(userId: 1);
           final message = eg.dmMessage(from: eg.selfUser, to: [user]);
           await setupPage(tester,
@@ -271,7 +271,7 @@ void main() {
           checkTitle(tester, '${user0.fullName}, ${user1.fullName}');
         });
 
-        testWidgets('no error when one user somehow missing from store.users', (tester) async {
+        testWidgets('no error when one user somehow missing from user store', (tester) async {
           final users = usersList(2);
           final user0 = users[0];
           final user1 = users[1];