Allow admins to disable Synchrony on a page-by-page basis

Give Confluence Admins the ability to disable Collaborative Editing on certain pages. Potentially multiple modes:

• Full Collaborative Editing
• Exclusive Single User Edit (with Synchrony)
• Exclusive Single User Edit (without Synchrony)

On some pages it may not be optimal to have multiple users changing them at once, or there may be a problem with Synchrony on that page. Disabling Synchrony for the whole space just fix some pages is overkill.