Improvements to impending cards w/Dances Up Earthquake

[samsullivan]

• many-to-many gameplayer:card relationship now includes energy and in_play
  • requires a migration, but should copy everything over from the old DB schema (tested)
• support adding/removing energy from impended cards
• support playing impended cards (when fully paid) so energy isn't paid for twice
• support unimpending (not normally a thing, but neither is unplaying) and forgetting cards from impending