Adds tracking of links in any HTMLText fields which reference SiteTree or File items.

Attaching this to any DataObject will add four fields which contain all links to SiteTree and File items
referenced in any HTMLText fields, and two booleans to indicate if there are any broken links. Call
augmentSyncLinkTracking to update those fields with any changes to those fields.
Note that since both SiteTree and File are versioned, LinkTracking and ImageTracking will
only be enabled for the Stage record.
for the extension applied to