Add flag for comments, to mark them compatible with the reaction system

Add flag for comments, to mark them compatible with the reaction system
Comments of different object types do not have their own LikeableProvider, but are all controlled by a central provider.  Therefore it is difficult to mark the comments as compatible to the reaction system. Therefore, we introduce a new flag that allows the global provider to control whether the comment is already compatible with the new system or not. If the "supportReactions" flag with the value "1" is passed to the object type, the notification of a reaction to the comment is sent. Otherwise the notification will not be sent.
See #2508