Move submission -> node relationship from 'entity_id' to 'nid' key.

A webform submission may be submitted to different entity types. Currently we only support adding a relationship from submission to the node where it was submitted to (no other entity types are support for such relationship).

This relationship resides within 'entity_id' key in the views data. Since 'entity_id' can represent literally any kind of entity type, that's a too general key for such relationship. It should be moved to its own dedicated 'nid' or 'node' key. We also need a hook update to update the relationship data in all the existing views.

Along the way, since messing with these relationships and granting a dedicated key for nodes, it might be worthwhile to exlore the option of adding relationship to any content based entity type.