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

mocks/include/mocks/EmptyApplication.hpp

@@ -94,6 +94,11 @@ public:
     {
         return nullptr;
     }
+
+    SeventvAPI *getSeventvAPI() override
+    {
+        return nullptr;
+    }
 };
 
 }  // namespace chatterino::mock