Add Stelo support

[marionbarker]

This was tested with a Stelo by the user who told me Stelo uses the DX01 prefix.

This PR builds and the following graphics are seen with the build.
There was also a suggestion that a single prefix of "DX" might be sufficient.

Edited with updated figure after commit: 8b18b0f: add warning about no glucose alerts for Stelo

[elnjensen]

@marionbarker, would it be worth splitting Stelo off as a separate menu entry, so we could know if people are using that (vs. G7/One+), and then pop up a warning about them not getting BG alerts with Stelo? (And possibly about this being untested with Loop.) That's not needed for pairing, but maybe worth a heads-up to the user?

[marionbarker]

This was tested successfully yesterday by a Stelo user who sent me a PM.

I will update the text to include that alerts and alarms are not available for Stelo and are provided by the Dexcom apps for G7 and ONE+.