[BUGFIX] Prevent data-htmlarea-external link attribute

The JavaScript method that inserts the link to the RTE (link_spec()) was
replaced with our own method (link_linkhandler()) that does not insert
the "data-htmlarea-external" attribute in the RTE.

Additinally a hook was implemented for the RteHtmlParser class to
prevent the attribute from being re-inserted by the parser becuase
it does not recognize the link type.