BUGFIX: HTML markup in ExtraMeta field being stripped out.

Refs silverstripe/silverstripe-cms#804.

Relies on https://github.com/silverstripe/silverstripe-framework/pull/2268. Link and image tracking should probably be moved out of HTMLEditorField and into silverstripe-cms at some point. Also, I am not currently set up with complete support for unit tests so this has undergone some manual testing but not unit testing.