More Webtoon Fixes (#406)

* Fixed an issue where "blank" images could occur at the end of a webtoon if the users scrolls up and down when near the last page, due to a gap in checking if prefetch range is within totalPages bound.

* Fixed goto first/last page not triggering webtoon reader to move. Disabled reading direction when in webtoon reader.

* Added some DOM Purging, images that are +/- bufferPages (5) from you will no longer reside in DOM. This helps on low end phones.

UI/Web/src/app/manga-reader/manga-reader.component.ts

@@ -849,6 +849,7 @@ export class MangaReaderComponent implements OnInit, AfterViewInit, OnDestroy {
     }
 
     this.setPageNum(page);
+    this.goToPageEvent.next(page);
     this.render();
   }