Use a marshmallow schema for regfom submissions

[tomasr8]

Adds mm_field_class and mm_field_kwargs to RegistrationFormFieldBase
which are used to dynamically construct the regform schema.

What works:

• regform submission (normal and from management)
• Incorporating field validators into the marshmallow schema

What doesn't work yet:

• Modifying existing registrations

[ThiefMaster]

Modifying existing registrations

not implemented anywhere in the new react code, so i'd just ignore that for now

[ThiefMaster]

Incorporating field validators into the marshmallow schema (including csrf_token)

csrf_token can go away, it's not needed. we have global csrf checks for all requests that aren't GET

for field validation you could just pass validate=... when defining the field itself; no need to add methods to the class dynamically

[ThiefMaster]

Check indico#4642 for the rationale why it was added - so I think in the end we should probably provide a similar signal that has access to the schema in order to modify that one if needed. But I think we could consider getting rid of it; if the UN folks (cc @OmeGak) still need it when adapting to the react regform (I'd still like to provide some way for plugins to register new regform field types!) we can see what's the best way of restoring it (or let them send another PR)

[OmeGak]

We are, indeed, still needing this signal (or something equivalent). You can see here how we are using it:
https://github.com/UNOG-Indico/indico-un/blob/master/un/modules/registration/forms.py#L60-L61