FEATURE: Submit post from mobile composer preview (PR #10286)

This will bring the behavior of the Submit From Preview mobile theme component into core.

Before before

After after

Behavior Notes

  • A bottom-border has been added to the preview to effectively create a “button footer”. I think this makes the buttons much more readable.
  • Submitting a post that does not meet title, body, etc requirements will immediately close the preview, exposing the relevant requirement tips that are normally displayed in the composer.
  • While the preview is open, the auto-save checkmark may display next to the submit button as it would when the preview is not displayed
  • The cancel/trash draft icons are suppressed while the preview is open. I don’t think we need to expose every button option in the preview.