shareMultitrack:toGroups:

+ (void)shareMultitrack:(MUSMultitrack *)multitrackToShare toGroups:(id<MUSSubscriptable>)sharedToGroups {
    NSMutableArray *newTrackLinks = [NSMutableArray array];
    NSMutableArray *sharedMultitracks = [NSMutableArray arrayWithCapacity:sharedToGroups.count];

    // Copy track links and multitracks, but do not copy tracks to avoid duplicating recording files.

    [MUSLocalStorageService writeTransaction:^{
        for (MUSGroup *group in sharedToGroups) {
            MUSMultitrack *sharedMultitrack = [group createNewMultitrackInGroup];
            [multitrackToShare duplicateShareableAttributesToMultitrack:sharedMultitrack];
            [sharedMultitracks addObject:sharedMultitrack];

            NSArray<MUSTrackLink *> *duplicatedTrackLinks = [multitrackToShare duplicateTrackLinksToMultitrack:sharedMultitrack];
            [newTrackLinks addObjectsFromArray:duplicatedTrackLinks];
        }
    }];

    [MUSLocalStorageService saveNewMultitracks:sharedMultitracks];
    [MUSLocalStorageService saveNewTrackLinks:newTrackLinks];
}