Use new Media Modal in Header Image selection UI

[tomauger]

The current Header Image selection UI should adopt the new media modal and allow:

• multiple simultaneous uploads
• removal of images from the "available header images" pool

Further, once we have sorted out the whole intermediate image size cropping issue, (ie: opening up cropping to more than just the thumbnail / all sizes) selecting an image for a header image should no longer create a new attachment for the header image, but just use the correct image size (assuming it's defined by the theme) and jump straight to the appropriate crop aspect ratio.

See #20516.

[SergeyBiryukov]

Related: #21819, #24678, #24679.

[celloexpressions]

This was implemented in the Customizer in 3.9 (#21785), the separate admin page is going away soon, and all of the other suggestions here are other existing tickets (that SergeyBiryukov linked).