The few other cases that used SimpleConfig.get_instance() now
either get passed a config instance, or they try to get a reference
to something else that has a reference to a config.
(see lnsweep, qt/qrcodewidget, qt/qrtextedit)
When encoding transactions as QR codes, we encode the tx bytes as base43
first. This makes it easier to load a transaction if the user manually
decodes the QR code.