ENH Improve quality of thumbnails in assetadmin and uploadfield

[GuySartorelli]

Description

• Improves the quality of thumbnails in the tile view of AssetAdmin and in UploadField instances.
• Makes the grid view of AssetAdmin use the same thumbnails as the tile view.
• Updates the thumbnail in AssetAdmin if the thumbnail method has changed since the thumbnail was last generated.

See #1224 for more details - there was a lot of discussion in there.

Manual testing steps

Before pulling this PR, add a bunch of different images to asset admin, and into upload fields.
Then pull this PR.
Check asset admin (both tile and gallery view) - you should see the new thumbnails.
Check uploadfield - you should see the new thumbnails there too.

If the thumbnails were blurry before the PR, you should notice a stark improvement after pulling the PR.

Issues

• UploadField and AssetAdmin gallery-view thumnails are blurry #1228

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)
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• 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]

Closing - we can reuse this code when we're ready to pull this through for CMS 6.
We'll need to provide a clear way (e.g. a BuildTask) for projects upgrading to generate their new thumbnails in a way that doesn't require sitting and waiting for them to regenerate for each page of images you visit in the asset admin.