Fix redirection when registering for existing organization which has autoVerify=true

[philippotto]

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• needs to be tested locally
• set autoverify=true for an organization
• use onboarding flow, select that organization, click "join", and register --> you should be redirected to the dashboard
• when using the onboarding flow to create a new organization, everything should work as usual

Issues:

• fixes Onboarding is broken for "Demo Lab" #3971

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• Updated changelog
• Ready for review

[daniel-wer]

Nice fix! One could also think about fixing this in the router.js with a <Redirect ..., but this works equally well I'd say :)

[philippotto]

One could also think about fixing this in the router.js with a <Redirect ...,

That was my first try, but then the onboarding flow couldn't be completed, because store changes (e.g., change of active use), triggered a re-render in the router which led to an early redirect. So, I had to use the explicit componentDidMount lifecycle :)