Track outgoing links

Currently, we do not register when a user clicks on a link to another page (say, information about the project, or a link to additional data / similar projects).
Similar to how google and facebook do it, this change makes all links in static page go to an intermediary redirect site, and then get redirected to the final URL.
Fixes hhucn/adhocracy.hhu_theme#341.
Merge remote branch 'origin/develop' into track-outgoing-links
[track outgoing links] Move into own library and apply everywhere
[track outgoing links] Add actual library (oops!)
[track outgoing links] Do not rewrite links to /example, only to //example.net/
Merge pull request #437 from phihag/track-outgoing-links

Track outgoing links