autocomplete: Exclude user from autocomplete popup if deactivated

Deactivated users can not be mentioned, so this commit ensures that inactive users are excluded from the list that gets rendered in autocomplete popup.
Fixes #451

Author: Khader-1

lib/model/autocomplete.dart

@@ -289,6 +289,8 @@ class MentionAutocompleteQuery {
     // TODO(#236) test email too, not just name
     // TODO(#237) test with diacritics stripped, where appropriate
 
+    if (!user.isActive) return false;
+
     return _testName(user, cache);
   }
 

test/model/autocomplete_test.dart

@@ -344,6 +344,10 @@ void main() {
       doCheck('Four F', eg.user(fullName: 'Full Name Four Words'), false);
     });
 
+    test('user is always excluded when not active regardless of other criteria', () {
+      doCheck('Full Name', eg.user(fullName: 'Full Name', isActive: false), false);
+    });
+
   });
 
 }