Move margin bottom of the form section to the close editor snippet button

[igorschoester]

Summary

This PR can be summarized in the following changelog entry:

• Moves the margin bottom from the FormSections to the CloseSnippetEditor button.

Relevant technical choices:

• N/A

Test instructions

This PR can be tested by following these steps:

• Ensure the snippet editor margins / paddings look the same on the metabox.
• On the settings pages this will solve a to-much-margin-at-the-bottom error from @jcomack

[boblinthorst]

CR: ok 👍

[boblinthorst]

Acceptance: ok 👍. Looks good on a post