Author Color Fixes

Author color is now cached so that it will not be re-calculated
each time a new message is posted.  This cache gets cleared when
the theme is changed.

Additionally, the author color is now automatically refreshed
when the theme is changed, fixing the issue where you had to
change rooms before the colors would switch.

src/MainWindow.cpp

@@ -112,7 +112,11 @@ MainWindow::MainWindow(QWidget *parent)
 
         connect(
           userSettingsPage_, SIGNAL(trayOptionChanged(bool)), trayIcon_, SLOT(setVisible(bool)));
-
+        connect(userSettingsPage_, &UserSettingsPage::themeChanged, this, [this]() {
+                utils::clearAuthorColors();
+        });
+        connect(
+          userSettingsPage_, &UserSettingsPage::themeChanged, chat_page_, &ChatPage::themeChanged);
         connect(trayIcon_,
                 SIGNAL(activated(QSystemTrayIcon::ActivationReason)),
                 this,