feat(preferences): allow hiding of vacation the vacation period

This is controllable using the new SOGoVacationPeriodEnabled
domain defaults variable.

SoObjects/SOGo/SOGoDomainDefaults.h

@@ -52,6 +52,7 @@
 - (int) forwardConstraints;
 - (NSArray *) forwardConstraintsDomains;
 - (BOOL) vacationEnabled;
+- (BOOL) vacationPeriodEnabled;
 - (NSString *) vacationDefaultSubject;
 - (NSString *) vacationHeaderTemplateFile;
 - (NSString *) vacationFooterTemplateFile;