Support for selectively hidden form fields

This new functionality will be used to hide form fields
not supported by the ISPConfig backend (i.e. vacation subject
is not user configurable in ISPConfig).

Signed-off-by: Jan Schneider <jan@horde.org>
Style
[jan] Hide unsupported form fields of vacation form (Michael Bunk <mb@computer-leipzig.com>).
Sieve doesn't support 'excludes' and 'ignorelist'.
Ingo backend for ISPConfig-Support

Signed-off-by: Jan Schneider <jan@horde.org>
ISPConfig supports sharing, because it uses a central admin user.
Style
[jan] Add vacation driver for ISPConfig (Michael Bunk <mb@computer-leipzig.com>).