Make date_of_birth optional on the user model

[lukasrad02]

Is your feature request related to a problem? Please describe.
If Ephios is using an OIDC IDP that does not provide a claim for the date of birth, this field is not set for all users created trough this IDP. However, since date_of_birth is listed in the required fields, such a user cannot be edited without setting the birthday. When editing a user, the birthday might not be known, so one has to guess it to be able to submit the form.

Describe the solution you'd like
Allow changing a user without specifying a date of birth if it wasn't specified for this user before. The easiest way would be to make the date_of_birth field optional on the UserProfile model.

Describe alternatives you've considered

• Leave the date_of_birth field required by default, but make it optional when editing users that don't have a date of birth set yet.
• Leave the date_of_birth field required by default, but make it optional for all user accounts coming from a specific identity provider (configurable) or any identity provider.
• Allow an administrator to configure whether the date of birth is required or not (for all users).