Calibrate the light intensity in the backend

[sonnyp]

Follow up to #941

Motivation

Current we calibrate light intensity in the browser using Node-Red.

There are a couple of issues with it:

• the preview and the still camera streams will always differ
• graphic stack can affect pixel reading https://superuser.com/questions/1904930/why-is-the-same-color-different-on-each-display

In general we must not read or infer from the preview stream. It is made for operations, not analysis/calibration.

ToDo

• Calibrate using real captures in the backend