services: Add quassel.

* gnu/services/messaging.scm (<quassel-configuration>): New record type.
(%quassel-account, %quassel-activation): New procedures.
(quassel-service-type): New variable.
* gnu/tests/messaging.scm (%test-quassel): New variable.
(run-quassel-test): New procedure.
* doc/guix.texi (Messaging): Document quassel service.