Revert "_includes: relative URLs"

This reverts commit 6870eb581d6ceaa99f33813819246f5930049332.  It
broke URLs on bugs.xcsoar.org and forum.xcsoar.org.
UNIX text format
config, layouts, templates: use absolute URL only for TRAC and forum
new forum and TRAC links without redirect
release v6.6_alpha2
command "new_post"
post: XCSoar 6.5.4 released
post: XCSoar 6.6_beta1 released
post: XCSoar on the Aero Expo 2013
_includes/header: fix server_url for TRAC and forum
post: XCSoar 6.6_beta2 released
XCSoar 6.6_beta2, missing change to config.yml
post: XCSoar 6.6 released
footer: copyright year 2013
new top-level "contact" page

Replaces the navigation in "discover".  This is not about
"discovering" XCSoar, but about getting help after the user has
already discovered XCSoar.
new "hardware" page

Lessons learned from the AERO2013.  Give users some guidance.
posts: replace contact links
hardware: green color, just like download
css: missing underline colors for hardware and contact
discover/irc: move to contact, adjust navigation