Implement Restore Revision dialog iOS

[yaelirub]

For discovery history - wordpress-mobile/WordPress-Android#10008, wordpress-mobile/WordPress-Android#9812
If the post has no local changes and a remote autosave, we are going to show a dialog to the user.
This issue is for implementing the design for this dialog.

Matching Android Issue:

wordpress-mobile/WordPress-Android#10108

Known Tasks

• Add Tracks events as described in Add Tracks to the new Unpublished Revision dialog WordPress-Android#10444

###UPDATE - 10/21/19
the acceptance criteria has evolved through a couple of Android issues and pull requests:

• iOS will use the system alert (pending design approval) for the dialog
• Posts containing autosaves on the post list screen will display the label "You’ve made unsaved changes to this post", this label previously read "Unhandled Auto Save"
• Even after tapping on the “Restore” button, users can still back out of the post without editing - and if they tap edit again they will be presented with the dialog (this will be done via the "Discard Changes" button, any unsaved changes are discarded)

[shiki]

@guarani will be working on this.

[shiki]

Moved this out of the Offline Posting board to keep the board focused on “posting”.