Add option for username tab completion only with an @ prefix (#1573)

* add an additional check for username completion without an @ prefix

* Changed setting name

* default userCompletionOnlyWithAt to false

src/common/CompletionModel.cpp

@@ -127,7 +127,7 @@ void CompletionModel::refresh(const QString &prefix, bool isFirstWord)
                               TaggedString::Type::Username);
                 }
             }
-            else
+            else if (!getSettings()->userCompletionOnlyWithAt)
             {
                 for (const auto &name :
                      usernames->subrange(Prefix(usernamePrefix)))