Before it would be applied on the fly during the rendering of the
HtmlEditorField, and only be written to the database during the
subsequent write.
We just shift the behaviour to apply just-in-time.
This code has a dependency on SiteTree, so it fits much better in the
cms module.
Abstracted away the content parser so the same code can be reused both
in the render phase (to highlight the links) and in the write phase
(storing information about broken links and references).
