feat: make marketing email opt-in checkbox selectively ignorable

[iloveagent57]

ported from #38022

We want to support a flow for SSO-enabled Enterprise customers who have agreed off-platform that none of their learners will opt-in to marketing emails or sharing research data. This change proposes to do so by adding an optional field that, when enabled, disables the presence of the two checkboxes on this registration form and sets their values to false.

Local Testing

See the added doc on how to test locally. Do one pass with the new model option disabled and private.py settings that have

ENABLE_THIRD_PARTY_AUTH = True
MARKETING_EMAILS_OPT_IN = True

Visit http://localhost:18000/auth/idp_redirect/saml-default, be redirected to mocksaml.com, then click "Sign In".

Now change your provider config to set "Skip registration optional checkboxes" to true.
Visit http://localhost:18000/auth/idp_redirect/saml-default again, be redirected to mocksaml.com, then click "Sign In".

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[kdmccormick]

Is there any supporting information you can link to?

[iloveagent57]

@kdmccormick Sorry for posting with empty description, I just updated it with what this code intends to support. good call on those exception handlers, we can clean that up.