Create a content provider instance to get message content models from

This means that all content models will now come from the same source to remove duplication across multiple models and `chatbarcaches`.

It also handily breaks the dependency on needing `MessageContentModel` for `NeochatRoom`

autotests/reactionmodeltest.cpp

@@ -9,6 +9,7 @@
 
 #include <Quotient/events/roommessageevent.h>
 
+#include "models/messagecontentmodel.h"
 #include "testutils.h"
 
 using namespace Quotient;