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/eventhandler.h

@@ -241,15 +241,6 @@ public:
      */
     QSharedPointer<LinkPreviewer> getLinkPreviewer() const;
 
-    /**
-     * @brief Return a ReactionModel object for the event.
-     *
-     * A nullptr will be returned for any event that doesn't have any links so the
-     * return should be null checked and an empty QVariantList (or other suitable
-     * empty mode) provided if null.
-     */
-    QSharedPointer<ReactionModel> getReactions() const;
-
     /**
      * @brief Whether the event is a reply to another in the timeline.
      */