Provide an opt-out modal confirmation when saving changes to content that is not configured to hold revisions.

This is related to #1842036: [META] Convert all confirm forms to use modal dialog → and I consider it to be a follow-up task...

It seems that there is consensus to move the "delete" tab back to being a button and adding a modal to it that will hold the confirmation message. This is precautionary measure in order to prevent data loss from accidental clicks to the "delete" button.

Now, when it comes to the "save" button, when a content type is not configured to retain any old revisions (which is the current default), it is subject to the same accidental data loss. This does not apply to the "publish"/"unpublish" action that can be safely reverted.

I believe that we should treat this case with the same precaution as the "delete" action and provide a modal confirmation. If people think that this might be obstructive, then perhaps the dialog could have a "do not show this warning again" opt-out.

Related issues/articles:
#482312: Proposal: unpublish rather than delete content → (against d.o infra)
#1608322: Configuration to unpublish comment instead of comment deletion → (against the Comment goodness contrib module)

Unpublish, don't delete
Can I recover a missing content page?

Related projects:
Content unpublish
Soft delete project page