Caches should be scoped to controllers/singletons

[Nerixyz]

Currently, we have some mutable static caches (mainly for emotes/images).

• chatterino2/src/providers/ffz/FfzEmotes.cpp

  Line 62 in 4d9ea37

  static std::unordered_map<EmoteId, std::weak_ptr<const Emote>> cache;

• chatterino2/src/providers/bttv/BttvEmotes.cpp

  Line 54 in 4d9ea37

  static std::unordered_map<EmoteId, std::weak_ptr<const Emote>> cache;

• chatterino2/src/providers/seventv/SeventvEmotes.cpp

  Line 51 in 4d9ea37

  static std::unordered_map<EmoteId, std::weak_ptr<const Emote>> cache;

• chatterino2/src/messages/Image.cpp

  Line 288 in 4d9ea37

  static std::unordered_map<Url, std::weak_ptr<Image>> cache;

---

chatterino2/src/providers/twitch/TwitchEmotes.hpp

Lines 99 to 100 in 4d9ea37

	UniqueAccess<std::unordered_map<EmoteId, std::weak_ptr<Emote>>>
	twitchEmotesCache_;

This is a good start, but we should go further and periodically clean up these tables (or have some smarter algorithm). It's noteworthy, that weak_ptrs to shared_ptrs allocated with make_shared prevent the underlying storage from being freed when all shared_ptrs go out of scope (the destructor still runs, but the storage is only cleaned up later, see blog).

Loosely related to #2732.