[BB-200] add validators for color fields in the registration form
Created by: dupainaulevain
Followup of https://github.com/open-craft/opencraft/pull/402
Implementation notes
The regex validation only happens when the form is submitted for the same reason [subdomain needs a special patch|https://github.com/open-craft/opencraft/blob/914b759abd4e99c48f3514177b34e0436898dc47/registration/forms.py#L180]. This is not a problem however since this validation is only a safeguard against the very rare case where someone (or a program) fills the registration form without the help of a color picker.
Instructions to test manually:
- make run.dev
- firefox http://localhost:5000/registration
- remove type="color" from one color field (right click + inspect)
- enter #1234 in the field revealed
- enter all required values in the form
- click the submit button
- check there is an error message under the field looking like this: