Update deprecated links and open external links in new browser window

* Contact, imprint and fortiss homepage are still opened in the same
  browser window
* Update download link for GCC and two referenced papers
* Update outdated links where possible, remove otherwise

Issue-Ref: 4031
Issue-Url: https://af3-developer.fortiss.org/issues/4031



Signed-off-by: Simon Barner <barner@fortiss.org>