Specify a custom vacation subject

User can now specify a custom vacation subject. For Sieve servers
implementing the variables extension, one can write ${subject} to insert
the original subject in the auto reply.

SOGoDefaultVacationSubject is a new domain defaults parameter used when
the user doesn't specify a custom subject.

Fixes #685, #1447

SoObjects/SOGo/SOGoDomainDefaults.h

@@ -52,6 +52,7 @@
 - (BOOL) forwardEnabled;
 - (int) forwardConstraints;
 - (BOOL) vacationEnabled;
+- (NSString *) vacationDefaultSubject;
 - (NSString *) vacationHeaderTemplateFile;
 - (NSString *) vacationFooterTemplateFile;
 - (NSString *) mailingMechanism;