AO3-6860 Allow CSS aspect-ratio property in skins

[Taxen99]

Issue

https://otwarchive.atlassian.net/browse/AO3-6860

Purpose

This allows the aspect-ratio CSS property to be used in skins, and updates the help popup for skins accordingly.

Credit

Taxen99 (he/him)

[sarken]

Hi, Taxen99!

Thank you so much for this pull request. Someone will be along soon to do a thorough review, but I noticed some automated test failures related to the i18n changes and wanted to give you a quick heads up that our i18n-tasks documentation might help you fix those up.

In the meantime, we've updated the Jira issue status to In Review so no one mistakenly creates a duplicate pull request. If you'd like the ability to comment on, assign, and transition issues in the future, you're welcome to create a Jira account! It makes things a bit easier for us on the organizational side if the Full Name on your Jira account either closely matches the name you'd like us to credit in the release notes or includes it in parentheses, e.g. "Nickname (CREDIT NAME)."

Once you've done that (or if you've already done it -- Jira has been unreliable about showing us new accounts in the admin panel lately), you can either reply here or send an email to otw-coders@transformativeworks.org with your account name and email address and we'll set up the permissions for you.

Thanks again for contributing! If you have any questions, you can contact us at the same email address listed above.