✨ Added drag-and-drop card re-ordering in the editor

[kevinansfield]

no issue

• add vertical drop position indicator handling to koenig-drag-drop-handler service
• fixed issues with nested drag-and-drop containers
• register card drag/drop handler in koenig-editor
  • add drag icon creation

TODO:

• add correct icon for embed and code cards
• hide card toolbar when dragging
• fix cursor to default when dragging
• do not trigger edit mode when dropping a card on itself
• add br-pill class to the drop indicator
• show image thumbnail when dragging an image card
• fix drop indicator not appearing after switching posts
• fix drop indicator position when first section in post is not a card
• fix card outline flicker whilst dragging over cards

[coveralls]

Coverage increased (+0.02%) to 71.357% when pulling a608e06 on kevinansfield:dnd-card-reorder into 58ab778 on TryGhost:master.