NightscoutAPIClient CGM service

[gestrich]

This adds Nightscout as a CGM plugin. (See Zulip discussion). The plugin appears to have been developed by ivalkou. Interface changes were required to make this work with the latest dev, along with some some small UI navigation updates.

Integration checklist

• Review and approve Loop PR 1540 (Cherry-picked in commit f98e742f739c92736a944fbe8eb27b442d40bc4a)
• Review and approve LoopKit PR 382 (Cherry-picked in commit ece225720eabe8dda38ebcf69180872eb50308c2)
• Review and approve Dexcom Share Client PR 28 (Cherry-picked in commit 9997d81b6adf2c07cfbe7783111d67251ccea033)
• Review and approve rilelink_ios PR 713
• Fork the NightscoutAPIClient repo into the LoopWorkspace account so we can host the modified branch.
• Review the NightscoutAPIClient changes (PR 2) that were required to work with latest dev.
• Merge

[ps2]

Why is there a kalman filter option on the client side? Seems like if a CGM needs filtering/smoothing, it should be on the data generation side.

[gestrich]

@ps2 I disabled the Kalman filter UI per that feedback. I'm not sure what source was being used that necessitated that but it doesn't seem we need it in the plugin. Let me know if any further questions on that.

[4gra]

The data xdripswift feeds into Nightscout is unfiltered, and this option corrects that in my testing. Seems like an OK use case to me, but maybe my use will be atypical.

[ps2]

Nightscout remote CGM has been merged.