Decide how to point/show users to the SDK from e-mails for appointment cancelling or payments

[joeribekker]

Questions:

• Do we want to use Django views or the SDK to show non-form pages (appointment cancel, payment)? The SDK has all the styling but can be hosted anywhere.
• How do we know where the SDK is hosted to point users to the right URL?

We need to think about this.

Possible solution

• [x] Store the form request URL on the submission (move from SubmissionPayment to Submission)
• [x] Remove global config of form URLs (if present)
• [x] Use the form request URL for where they are needed (see Where is this SDK URL needed?)
• [ ] Document the con's and how to mitigate this (make proper redirects)
• [x] Update SDK to send the window.location (?) as formUrl data value (@sergei-maertens )

Pro's

Works where-ever the form is shown (whether in a CMS or using the stand alone version.

Con's

• Moving the form between sending the email and the user clicking on the link, leads to an error

Where is this SDK URL needed?

• Change appointment
• Cancel appointment
• Save in between / Resume form later
• Payment completion page

All of these URLs are communicated via e-mail and users can click these whenever.

[sergei-maertens]

Some more thoughts in https://github.com/open-formulieren/open-forms/pull/804#discussion_r734422749

[sergei-maertens]

Re-open because docs :-)