Ghostscript is needed to export Gant diagrams as PDF. Thus, add the
option `services.redmine.components.ghostscript` allowing to enable or
disable the component. The component is disabled by default. Enabling
the option will add Ghostscript to the Redmine environment and configure
the setting `gs_command` in the Redmine configuration file.
Signed-off-by: Felix Singer <felixsinger@posteo.net>