3 new filters that aim to improve the extensibility of Press This:
1) `press_this_save_post_content` - Applied right after the side_load_images in order to allow potential side loads of other types of media.
Example use case: side load non-image media, such as audio or video.
2) `press_this_useful_html_elements`
Allows filtering of currently hard coded array of HTML elements allowed in fetch_source_html step for special cases where additional HTML elements need to be kept.
Example use case: HTML5 elements, such as amp-img, that someone wants to pull in.
3) `press_this_suggested_content`
A filter for the content right before it's passed to the editor and presented to the user.
Example use case is when someone stored posts in a different, non-HTML format, such as Markdown, this is essential.
Fixes#34455.
Props cadeyrn, kraftbj
Built from https://develop.svn.wordpress.org/trunk@36672
git-svn-id: http://core.svn.wordpress.org/trunk@36639 1a063a9b-81f0-0310-95a4-ce76da25c4cd
5c9d92b7f8