Refactor ConcurrentMap

* Add operator[] to ConcurrentMap which returns a TValue reference
* BTTV/FFZ channel emotes are now stored in the Emote Manager, and each Channel object has a reference to their own BTTV/FFZ channel emote map.
* Restructure EmoteManager a bit (simplify the ConcurrentMap havoc).
* Add EmoteData struct which can store emote data (for now only messages::LazyLoadedImage*)
* Add CompletionManager that does nothing

src/completionmanager.cpp

@@ -0,0 +1,20 @@
+#include "completionmanager.hpp"
+
+namespace chatterino {
+
+CompletionManager::CompletionManager()
+{
+}
+
+CompletionModel *CompletionManager::createModel(const std::string &channelName)
+{
+    CompletionModel *ret = new CompletionModel();
+
+    return ret;
+}
+
+void CompletionManager::updateModel(CompletionModel *model, const std::string &channelName)
+{
+}
+
+}  // namespace chatterino