Replace historical formstep recording bandaid with form snapshotting

[sergei-maertens]

Split from #3283, was framed as:

Proper solution for form step references?

open-forms/src/openforms/submissions/models/submission_step.py

Line 98 in 8f7b003

on_delete=RECORD_HISTORICAL_FORM_STEP,

Initial thoughts after discussion with Joeri:

• Store snapshot of formio component/form step configuration with submission once its completed
• Processing should only operate on snapshot data
• Be robust - $currentCode needs to take into account older versions in snapshot data
• Document procedures on how to fix failing registrations now that the form itself can no longer be updated to apply fixes
• Documented (intended) behaviour

Attaching this to milestone v4, but we hope to do this sooner. It's debatable whether this is a breaking change or not - considering what our intent is/was :)