Move 60px margin bottom from form to caption input

[Georges-GNM]

What's changed?

The standard article form has a 60px margin set to it (following this PR), but in hindsight we'd prefer to only display it if the slideshow is in place. The caption input, at the bottom of the slideshow, now has the property, as it only appears when there's a slideshow.

The article form can still have a margin bottom; this has been left in for feast related things.

Screenshots

Before - always 60px margin bottom:

mbbb.mov

After - 60px margin bottom only when showing the caption input:

mbaft.mov

Checklist

General

• 🤖 Relevant tests added
• ✅ CI checks / tests run locally
• 🔍 Checked on CODE

Client

• 🚫 No obvious console errors on the client (i.e. React dev mode errors)
• 🎛️ No regressions with existing user interactions (i.e. all existing buttons, inputs etc. work)
• 📷 Screenshots / GIFs of relevant UI changes included

[github-actions]

Deploy build 8654 of cms-fronts::facia-tool to CODE

All deployment options

• Deploy build 8654 of cms-fronts::facia-tool to CODE
• Deploy parts of build 8654 to CODE by previewing it first
• What's on CODE right now?

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From guardian/actions-riff-raff.