Pass the entity type to action and action config forms

Extracted from #1057158: Provide an action for modifying entity values.
Some actions (like that one) need to know which entity type they are operating on.
The entity type should be passed to the action form as part of $context, and to the action config form (embed in the VBO settings) as the second param.
I was thinking of adding $context to the action config form callback, but seems pointless since I can't think of anything else we can pass at that stage.