Make local unread markers optional, off by default

Add RoomList.local_unread_markers option to settings.py,
which is False by default.
Before, this way always enabled and conflicts with push rules by marking
anything as unread without any logic.

src/gui/MainPane/RoomDelegate.qml

@@ -66,7 +66,7 @@ HTile {
                         (model.bookmarked ? "\u2665 " : "") +
                         (model.display_name || qsTr("Empty room"))
                     color:
-                        model.local_unreads ?
+                        model.unreads || model.local_unreads ?
                         theme.mainPane.listView.room.unreadName :
                         theme.mainPane.listView.room.name
                 }