Made structural changes to server-side Preference code based on feedback

store/store.go

@@ -152,8 +152,6 @@ type WebhookStore interface {
 }
 
 type PreferenceStore interface {
-	Save(preference *model.Preference) StoreChannel
-	Update(preference *model.Preference) StoreChannel
-	SaveOrUpdate(preferences ...*model.Preference) StoreChannel
+	Save(preferences *model.Preferences) StoreChannel
 	GetByName(userId string, category string, name string) StoreChannel
 }