Make the whole right sidebar scrollable

The sidebar grew a lot and now contains some tools. This makes the space
left for the room members very small. Particuliary on mobile.

So make the whole sidebar contained inside the ScrollView

This also refactor a few things (e.g. use a plain ToolBar for the header)

src/qml/Panel/DirectChatDrawerHeader.qml

@@ -48,7 +48,7 @@ ColumnLayout {
                     room: room,
                     user: room.getUser(room.directChatRemoteUser.id),
                 })
-                popup.closed.connect(function() {
+                popup.closed.connect(() => {
                     userListItem.highlighted = false
                 })
                 if (roomDrawer.modal) {
@@ -61,7 +61,6 @@ ColumnLayout {
 
     Kirigami.Heading {
         Layout.fillWidth: true
-        level: 1
         type: Kirigami.Heading.Type.Primary
         wrapMode: QQC2.Label.Wrap
         text: room.displayName