Feature request: Add support for displaying in sats

[kaloudis]

See ElementsProject/lightning-charge#74 (comment)

[shesek]

While implementing this in lightning charge, I also changed the default display unit to sats. So lightning publisher connected to lightning charge v0.4.19+ should already display amount in sats.

What could be done in lightning publisher is making the display unit configurable via the GUI. It would require a new field in the admin settings page, and to create the invoice with checkout.display_unit added to the invoice metadata.

A PR for this would be gladly accepted!