Perform a full save when previewing drafts

[adamsilverstein]

Describe the bug
When clicking the preview button for draft posts, Gutenberg should save all post details, including meta fields and the featured image. Currently Gutenberg performs an autosave when previewing drafts, which includes title, content and excerpt and leaves out meta and featured image information.

To Reproduce
Steps to reproduce the behavior:

1. Start a new post, add a title and a featured image and click save draft
2. Change the featured image to a new image
3. Click the preview button
4. Reload the editor screen

Expected behavior
At step 3 we should see the newly selected featured image in the preview - instead we see the first/saved image.
At step 4 we should see the new featured image because it was saved when previewing; instead we see the original image.

Note: saving the full post information for drafts when previewing matches the current classic editor behavior.

Additional context
See #12037 (comment) which solves featured image issue for non draft posts
Related: #11409 (which does not resolve this issue)
Related featured image issue: #9151

[earnjam]

Sort of related to save vs autosave and previews: #7927
And some work is being done on that in #7953