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/providers/seventv/SeventvAPI.cpp

@@ -78,11 +78,5 @@ void SeventvAPI::updatePresence(const QString &twitchChannelID,
         .execute();
 }
 
-SeventvAPI &getSeventvAPI()
-{
-    static SeventvAPI instance;
-    return instance;
-}
-
 }  // namespace chatterino
 // NOLINTEND(readability-convert-member-functions-to-static)