[FEATURE] RTE: Use HTML5-compliant custom attribute for external url's

htmlArea RTE sets custom attribute "external" on links to external
url's so that they are preserved as external by the RTE
transformation, even though they may look internal.

Use custom attribute "data-htmlarea-external" instead in order to be
HTML5-compliant.

Change-Id: I840f32bc6929debbe92d82731b8afdbec638dc07
Resolves: #34114
Releases: 4.7
Reviewed-on: http://review.typo3.org/9124
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Steffen Ritter
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland