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)
  • 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