[SAC] [trac service] inter-trac links enabled

[Sandro Santilli]

For those projects that are still using the OSGeo trac,
I've just enabled inter-trac links.

This means you can reference tickets/wikipages/changesets of
any OSGeo project using trac from any other trac instance by
simply prepending the project name in the link, like:

  geos:changeset:2234
  osgeo:#45
  ubuntugis:wiki:WikiStart

The project name is the last portion of the trac entrypoint:

  https://trac.osgeo.org/postgis <--- "postgis"

I hope you'll find it useful. Next step will be figuring out a
way to automatically show the incoming link from the target :)

--strk;