Revert libxml2 to 2.8.0 and libxslt to 1.1.26

As the newer versions result in problems with diazo (non-terminating
requests / white pages), libxml2 and libxslt are reverted to the last
working versions.

Note: You have to remove eggs/lxml* and parts/lxml in order to be
effective (same goes for the previous commit).
Autojoin feedback instance by default

By default, configuration option `adhocracy.instances.autojoin` is now
set to `feedback`. This fixes liqd/adhocracy#130.

Additionally, information is added on how to add all users to the
feedback instance in existing installations.
Merge branch 'develop' into hhu