Old CGM battery data displayed

[jmarler]

Issue description:
When users switch from uploading using the android uploader to using Share2NS bridge, battery data for the CGM is no longer uploaded. However, the NS web site still shows a battery value that uses the last battery value sent before the switch. The manual workaround is to purge all battery data.

Expected behavior:
The NS web UI should either:

• Have an environment variable/config option to toggle the display of the battery data and icon
• Recognize that the battery data is older than the last valid data point, and after a certain delta has been reached, either suppress showing the battery icon completely, or indicate that the data is no longer available in some fashion.

Recreation steps:

• Upload data using NS uploader on Android (including CGM battery data)
• Switch to using Share2NS bridge
• Observe that battery data never changes from last reported value

[jasoncalabrese]

There are a couple of tricks to solve this, add a new mongo document with {uploaderBattery: false}, delete your devicestatus data, and I think newer versions of the bridge even do something with this.

[jasoncalabrese]

See nightscout/share2nightscout-bridge#10