When autosaving drafts, don't create revisions.

[adamsilverstein]

Description

When dispatching the autosave event, pass a query var, then skip revisions for that save event.

How Has This Been Tested?

Creating and saving draft posts, monitoring the database.

Types of changes

• when dispatching savePost from autosave add an option autoupdate=1
• when initiating a save, add a gutenberg_autosave query var if the autoupdate option is set
• filter the number of revisions for WordPress to create, set at 0 if the gutenberg_autosave query var is present.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows has proper inline documentation.

[adamsilverstein]

Closing in favor of #4156