Allow Signup using an QR-Code

[ja-fra]

Description

Currently, participants have to manually enter the registration-token on the signup screen. In some cases also the endpoint-url needs to be changed.
This is hidious and error-prone process.

Change

On the signup-screen, add a button to scan a QR-Code. Once the QR-Code was detected and decoded, the signup-procedure shall be triggered directly.

The QR-Code will have the following signature: <endpoint-url>/signup?token=<token>, e.g. https://example.com/api/v1/signup?token=QWERTZ

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Split from #246