New functions in `media.php`:
* `wp_get_attachment_image_srcset_array()` - Returns an array of image candidate string data used to build a `srcset` value for an attachment given an `$attachement_id` and `$size`.
* `wp_get_attachment_image_srcset()` - Returns the `srcset` value for an attachment given an `$attachement_id` and `$size`.
* `wp_get_attachment_image_sizes()` - Returns the `sizes` value for an attachment given an `$attachement_id` and `$size` and optional arguments used to alter its output.
* `wp_make_content_images_responsive()` - A display filter for adding `srcset` and `sizes` to images embedded in content.
* `wp_img_add_srcset_and_sizes()` - A utility function used by `wp_make_content_images_responsive()` to add `srcset` and `sizes` to a single `<img>` element.
Modifies existing core functions:
* Modify `wp_get_attachment_image()` so the HTML returned for an image includes `srcset` and `sizes`.
* Modify `get_media_embedded_in_content()` (sup, 3.6 leftover) by adding `<img>` to the list of accepted tags that can be matched in content. This is used in `wp_make_content_images_responsive()` to find all of the images embedded in content before passing them off to `wp_img_add_srcset_and_sizes()`.
Tests:
* Add a new factory method to `WP_UnitTest_Factory_For_Attachment` named `create_upload_object()`
* Adds unit tests
* Updates unit tests
Props joemcgill, tevko, jaspermdegroot, mdmcginn, barryceelen, peterwilsoncc, fsylum, wonderboymusic, chriscoyier, benjaminpick, jrfnl, #12kingkool68, janhenckens, ryanmarkel, side777, ryelle, wturrell, micahmills, mattbagwell, coliff, DrewAPicture.
See #33641.
git-svn-id: https://develop.svn.wordpress.org/trunk@34855 602fd350-edb4-49c9-b593-d223f7449a82
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