(note that this is based on top of #5238)

• Previously we had a getPageSummary() call where the optional Referer parameter came first for some reason, which required us to pass an explicit null everywhere this call was made. Let's make this parameter come second, along with all other optional parameters, so that any overloaded calls can be simplified.
• In the SavedPageSyncService where we were making the calls to retrieve the PageSummary, MediaList, and MobileHtml, those calls were being made serially, not in parallel. Let's use async{} to make these calls all at once, and speed up saving articles for offline usage.