MessageEventModel media info improvements

Create a `messageeventmodel` role for media info and reply media info that is a QMap with all the required data.

This replaces the MediaUrlRole, FileMimeTypeRole and the ContentTypeRole. The reply role no longer needs the content role.

This also ensures mxc urls are now generated for replies. All the media parameters will now have default values assigned in the model so the QML no longer needs to do this.