Disable hidden fields for all forms

[rambip]

:tophat: What? Why?

This PR add the "disabled" property to a few hidden fields in some specific forms. The goal is to have 100% uniformity in the codebase: if the field of a form is hidden, then it is disabled. See the issue for why I think it's important.

:pushpin: Related Issues

• Fixes #15464

Testing

1. Go to admin
2. Create a new participatory process
3. Create a new "metting" event
4. click on Type -> In person
5. inspect the page The input element "online_meeting_url should be hidden but not disabled. If you refresh the page, the element "meeting_registration_url" should be disabled to

All the other forms should work in the same way as before.

:camera: Screenshots

N/A

:hearts: Thank you!

[rambip]

Are all the failed test produced by my change ? I find it surprising given that I change only a few js files. But I'm a completely new contributor so it's possible. Thank's for the help and the time.