Username creation/generation

[a-steinert]

Description of the issue

If there is no username being delivered through regapp, there is no username being shown as a possible entry in the signup form.

Ideas of solution

• If an email address is, the username should be set as the first part of the email as default
• A random generated username could be suggested

Describe possible ideas for solution and evaluate advantages and disadvantages.

Workflow checklist

• I am aware of the workflow in CONTRIBUTING.md