feat(core): allow pre/appended Sieve scripts

Allow domain admins to now set Sieve scripts that can be prepended
and appended to all users' scripts. This can now be controller
using the SOGoSieveScriptHeaderTemplateFile and
SOGoSieveScriptFooterTemplateFile domain defaults.

SoObjects/SOGo/SOGoDomainDefaults.h

@@ -48,6 +48,8 @@
 - (BOOL) forceExternalLoginWithEmail;
 - (BOOL) externalAvatarsEnabled;
 - (BOOL) sieveScriptsEnabled;
+- (NSString *) sieveScriptHeaderTemplateFile;
+- (NSString *) sieveScriptFooterTemplateFile;
 - (BOOL) forwardEnabled;
 - (int) forwardConstraints;
 - (NSArray *) forwardConstraintsDomains;