Split playbook into roles

The local.yml playbook was split into the following roles:
- xfce-gui-minimal: Install and configure a minimal xfce gui + applications
- mpstools: Install the mpstools (patch them if needed) and add the PATH
- geany: Install geany and configure it to replace snavigator
- dfn-ca-cert: Install the DFN CA certificate into the system and into firefox
(this is needed to access certain h-da websites)
- h-da_util: Install the hda-util.sh + desktop launchers for easy access to
the university network
- misc-applications: Install a bunch of small useful applications

The default known_hosts was removed since it didn't seem to work reliably