fix: deadlock and use-after-free in tests (#4981)

* fix: use-after-free in settings

* refactor: put seventv api into a singleton

* chore: add changelog entry

* Add warning for when the 7TV load fails

src/singletons/Settings.hpp

@@ -66,6 +66,7 @@ enum UsernameRightClickBehavior : int {
 class Settings
 {
     static Settings *instance_;
+    Settings *prevInstance_ = nullptr;
 
 public:
     Settings(const QString &settingsDirectory);