Add ability to save active settings to flash

[ryan-summers]

Add functionality to save the currently-operational settings into device flash memory via the USB interface. This was identified in #857 (comment)

[jordens]

When playing with it I noticed that after the change to a shared settings struct the behavior is not anymore (after the very positive changes to shared settings) what I'd have expected (without reading docs/code).

• set saves "everything" (in the sense that what is doesn't save is unchanged from default). At least now it should save only what's been set. But arguably it should not save at all anymore (since it already affects live application settings).
• There is no way to "save the settings to flash" without actively setting one. Since over USB we now share the MQTT view, I'd want a save command to make the currently active (and visible) settings the initial settings.

I think I closed this too early.