Refactor reactions

Currently we effectively create the reactions list in EventHandler then pass that data into a model. This reworks the model so that we just pass in a room and an event and it grabs it's own data. This means that:
- the functions in event handler are no longer required
- the model can update itself to add/remove reactions so no need to handle that in MessageEventModel
- MessageEventModel only needs to create new ReactionModels or remove old ones when no reactions exist anymore

A basic test suite has also been created for the ReactionModel

src/models/messageeventmodel.h

@@ -131,6 +131,7 @@ private:
     QString lastReadEventId;
     QPersistentModelIndex m_lastReadEventIndex;
     int rowBelowInserted = -1;
+    bool resetting = false;
     bool movingEvent = false;
     KFormat m_format;