Improve Repeated Onboard Flow

[PSchmiedmayer]

Description

If a user currently logs out, they don't have the option to enter an invitation code to sign up with a different account.
In addition, a user that is already onboarded and has already provided a consent should not be asked to sign the consent document again.

Reproduction

Open the application, follow the onboarding flow and log out. It is not possible to enter an invitation code (and we might even want to show some other parts of the onboarding flow as well) and there is no way to skip the consent document flow.

Expected behavior

Ability to do a complete sign-up process and skip the consent document step in the scenario described above.

Additional context

It would be great to take some of the mechanisms from this app to the main Template App as well to improve some of the Spezi Account handling as well as improvements in the onboarding flow as the ones described here.

Code of Conduct

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