When support for PDFs is available, on upload,
render 'Thumbnail', 'Medium', 'Large', and 'Full' sizes of
the first page, and save them in attachment meta.
Use these renders within Add Media, Media Gallery and List views,
Attachment Details, Post/Attachment Edit screens, and Attachment pages.
Support available by default via Imagick -> ImageMagick -> Ghostscript,
but can be provided by any `WP_Image_Editor` that supports PDFs.
Props adamsilverstein, azaozz, celloexpressions, desrosj, dglingren, ericlewis, ipstenu, joemcgill, joyously, markoheijnen, melchoyce, mikeschroder, tomauger.
Fixes#31050.
git-svn-id: https://develop.svn.wordpress.org/trunk@38949 602fd350-edb4-49c9-b593-d223f7449a82
35e6dbe14f