Press This Structural Rewrite

[georgestephanis]

This is a ticket to tidy up and abstract some of the Press This code into wp-admin/includes/press-this.php to make it simpler to work with and use functionality in plugins and such.

Once this structural change is done, I've got a couple enhancements coming down the pipes including reading OG/Twitter/Meta tags from the source page, simpler upgrades to the JS (jorbin's suggestion) better image pulling, etc.

This patch below, however, is merely structural, not adding in any new functionality, merely tidying up what we've got so far.

#18806 was marked as a duplicate.

Related: #23278