Remove SubmissionValueVariable.form_variable FK field to avoid confusion

[sergei-maertens]

Closes #5058

Changes

• Updated factory to ensure that a form variable exists for submission value variable (via factory)
• Updated tests to specify sufficient metadata for the form variables. I actually did a POC first where each variable was created explicitly before updating the factory to make sure there's no false confidence through unintended factory side effects.
• Removed the form_variable FK field as it's not needed and may otherwise cause confusion.

There's still an interesting cascade delete now in the code where a form definition can delete the related form variable, leading to a submission variable existing without the matching form variable being present. I think our code is robust enough to deal with this, but it's something that could cause issues in the future if the service layer is not used to process submission values.

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Dockerfile/scripts

  • Updated the Dockerfile with the necessary scripts from the ./bin folder

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.72%. Comparing base (3f79ba2) to head (c626d78).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5071   +/-   ##
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  Coverage   96.72%   96.72%           
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  Files         770      770           
  Lines       26517    26523    +6     
  Branches     3454     3454           
=======================================
+ Hits        25649    25655    +6     
  Misses        606      606           
  Partials      262      262           

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[stevenbal]

Why is this needed, can't we remove the property as well?

[sergei-maertens]

we're still setting this attribute dynamically and lots of code requires self.form_variable to be defined in some capacity. This type annotation declares that it is a known attribute with a particular expected type. Not defining this here would definitely fail the type checking in CI :)