Tabs

- Improved installer and updater for multiserver mode. Modules and plugins were only configured if the selected service is available on the server.

Moved server modules and plugins to new directory and updated the installer to create the symlinks for the modules and plugins from *-available to *-enabled directory.
Fixed a typo in mailfilter template.

Added a server module for clients and a mail plugin.

- improved several functions in site module
- added server classes to restart services