CERNAccess: show form when editing your registration

When allowing people to request access during registration, it would be good to also provide certain options afterwards

Some points that would be useful / should be considered:

- "Request access" after registering (maybe w/ a separate toggle off by default, since event organizers need to be informed about this since they have to grant access after someone requests it)
- Change/add car license plate number later (important for people w/ rental cars!)
- Modifying any of the extra personal data that's requested is probably not needed
- Do we want to ignore the general "can modify registration" logic w/ deadlines etc.? For the license plate for sure since the event organizer is not involved in such a change anyway.

Start by tracing the CERNAccess registration flow and how the existing form handles post-registration changes. The issue needs decisions on later access requests, license-plate edits, extra personal data, and deadline rules before implementation can be considered complete.

Written by the indexing model from the issue text.