[#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

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

• Release management

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

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

  • [x] Ran ./bin/makemessages_js.sh
  • [x] Ran ./bin/compilemessages_js.sh

• Commit hygiene

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.61%. Comparing base (ff04630) to head (4b50efe). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4683 +/- ## ======================================= Coverage 96.61% 96.61% ======================================= Files 746 746 Lines 25144 25145 +1 Branches 3302 3302 ======================================= + Hits 24292 24293 +1 Misses 589 589 Partials 263 263 ```

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[sergei-maertens]

Backports:

• stable/2.8.x: 92b01602b2767c66d518f54d8abe7a4a10e3479e
• stable/2.7.x: 94f36ab0c247fbba72066fcfffc29b236d25aff6

Older versions aren't affected