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.m

@@ -220,6 +220,11 @@
   return [self boolForKey: @"SOGoVacationEnabled"];
 }
 
+- (NSString *) vacationDefaultSubject
+{
+  return [self stringForKey: @"SOGoVacationDefaultSubject"];
+}
+
 - (NSString *) vacationHeaderTemplateFile
 {
   return [self stringForKey: @"SOGoVacationHeaderTemplateFile"];