[#4656] Make parent's serializer context available to the children (FileSerializer)

[vaszig]

Closes #4656

Changes

• Added regression test for the editgrid (didn't make context available to the children)
• Made parent's serializer context available to children (EditGrid)
• The other nested layout components work with files as expected (fieldset and columns)

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

• Commit hygiene

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