[backend] add effective_confidence_level computed field to users (#4304)

[labo-flg]

[To merge in feature branch for #4304]

This PR brings the effective_confidence_level to users.
It's a computed value (algo described in code comments) and not an attribute in the schema, that depends on user's confidence level and the confidence levels of his organizations and groups.

We also add platform_critical_alerts as part of settings, if any a dialog is shown to admins at login / any refresh of the app.
It invite admin to do something about an issue in the app with clear CTA. Only one alert possible: null effective confidence level found in some groups.

Further comments

Backend code was split to allow for easy unit tests, added to user-test.ts.

Note that adding group_confidence_level and org_confidence_level is not part of this PR, but the algorithm will work fine as this will be optional fields anyway.

[codecov]

Codecov Report

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Additional details and impacted files

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