fix: prevent CMS requirements from leaking into preview email

[wilr]

Description

When previewing an email template, the current behaviour is to include existing CMS styles which can be misleading.

Manual testing steps

• Create a HTML template for the email recipient
• Notice that the HTML template includes CSS when previewing.

Issues

• CMS requirements from leaking into preview email #1300

Pull request checklist

• The target branch is correct
  • See picking the right version
• All commits are relevant to the purpose of the PR (e.g. no debug statements, unrelated refactoring, or arbitrary linting)
  • Small amounts of additional linting are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
• The commit messages follow our commit message guidelines
• The PR follows our contribution guidelines
• Code changes follow our coding conventions
• This change is covered with tests (or tests aren't necessary for this change)
• Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• CI is green

[GuySartorelli]

I've added back the spot from the pull request template where a link to a issue needs to go. Can you please link to an issue? You'll need to create one if there isn't one already.

This is important because it lets us track the issue in Zenhub, which the CMS Squad uses to keep track of the progress of issues. We don't track PRs, so this may not every be reviewed properly if there's no issue for us to track.

[GuySartorelli]

Please also retarget this to the 6.2 branch so it can be released as a patch. You may need to reset your commits after retargetting the PR.

[wilr]

Rebased on 6.2 now!

[wilr]

This is important because it lets us track the issue in Zenhub, which the CMS Squad uses to keep track of the progress of issues. We don't track PRs, so this may not every be reviewed properly if there's no issue for us to track.

Ah another reason Zenhub should die! Far too much duplication

[GuySartorelli]

Sorry it's taken so long to review this. Mostly looks good, but should call Requirements::restore() after rendering the template.

[wilr]

Sorry it's taken so long to review this. Mostly looks good, but should call Requirements::restore() after rendering the template.

Done

[GuySartorelli]

LGTM, thanks for making that change