Update user colors to use Cache vs Utils

User colors are now stored in cache.  This is consistent
with other similar variables.  I think there's a bug
right now where it doesn't properly refresh colors
for the TimeLineItem when the theme is changed.

src/MainWindow.cpp

@@ -113,7 +113,7 @@ MainWindow::MainWindow(QWidget *parent)
         connect(
           userSettingsPage_, SIGNAL(trayOptionChanged(bool)), trayIcon_, SLOT(setVisible(bool)));
         connect(userSettingsPage_, &UserSettingsPage::themeChanged, this, []() {
-                utils::clearAuthorColors();
+                Cache::clearUserColors();
         });
         connect(
           userSettingsPage_, &UserSettingsPage::themeChanged, chat_page_, &ChatPage::themeChanged);