Improve User Avatar Model Roles

- Update the message event and user models so that the full source url is output.
- Separate the reply author into its own role
- Create an empty user object that can be passed so that the QML code no longer needs to check if certain parameters exist.
- Make avatarForMember return and empty QUrl if a valid avatar cannot be found and make use in the user and event models

As well as cleaning up the QML this should also stop the QML Image: Media id '' doesn't follow server/mediaId pattern spam in the log.

src/models/messageeventmodel.h

@@ -75,6 +75,7 @@ public:
         FileMimetypeIcon, /**< The icon name for the mime type of a file. */
 
         IsReplyRole, /**< Is the message a reply to another event. */
+        ReplyAuthor, /**< The author of the event that was replied to. */
         ReplyRole, /**< The content data of the message that was replied to. */
         ReplyIdRole, /**< The matrix ID of the message that was replied to. */
 
@@ -192,6 +193,8 @@ private:
     int refreshEventRoles(const QString &eventId, const QVector<int> &roles = {});
     void moveReadMarker(const QString &toEventId);
 
+    const Quotient::RoomEvent *getReplyForEvent(const Quotient::RoomEvent &event) const;
+
     std::vector<Quotient::event_ptr_tt<Quotient::RoomEvent>> m_extraEvents;
 
 Q_SIGNALS: