DUPP-572 Apply core class to get margins right

[enricobattocchi]

Context

• We want to fix the margins here:
  

Summary

This PR can be summarized in the following changelog entry:

• Fixes a bug where a section in the Classic Editor's submitbox would be displayed with incorrect margins.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• create a R&R copy and edit it
• check that the margins are now correct

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes DUPP-572

[maartenleenders]

🎉