Enable thumbnail rendering in the Resources ModelAdmin listing

[Scotchester]

This ModelAdmin thumbnail mixin either wasn't available in Wagtail when Resources was set up, or I just overlooked it.

Additions

• Added ThumbnailMixin to ResourceModelAdmin to enable actual rendering of the thumbnails for things like our placemats
• Adds a tiny bit of new global admin CSS through our first use of the global_admin_css hook in order to ensure that the column width fits the entire thumbnail

How to test this PR

1. Pull branch
2. Load up http://localhost:8000/admin/v1/resource/ and scroll down

Screenshots

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After

Checklist

• PR has an informative and human-readable title
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  • Consider prefixing, e.g., "Mega Menu: fix layout bug", or "Docs: Update Docker installation instructions".
• Changes are limited to a single goal (no scope creep)
• Code follows the standards laid out in the CFPB development guidelines