Webtoon Reader Fixup (#405)

* Navigate users to library page instead of home to prevent history block.

* Cleaned up the Contributing to describe new code structure

* Fixed a critical bug for how we find files for a chapter download (use ChapterId for lookup, not MangaFile.Id). Refactored how downloading works on the UI side to use the backend's filename whenever possible, else provide a custom name (and use backend's extension) for bundled downloads.

* Fixed a bug where scroll intersection wasn't working on books without a table of content, even though it should have.

* If user is using a direct url and hits an authentication guard, cache the url, allow authentication, then redirect them to said url

* Added a transaction for bookmarking due to a rare case (in dev machines) where bookmark progress can duplicate

* Re-enabled webtoon preference in reader settings. Refactored gotopage into it's own, dedicated handler to simplify logic.

* Moved the prefetching code to occur whenever the page number within infinite scroller changes. This results in an easier to understand functioning.

* Fixed isElementVisible() which was not properly calculating element visibility

* GoToPage going forwards is working as expected, going backwards is completly broken

* After performing a gotopage, make sure we update the scrolling direction based on the delta.

* Removed some stuff thats not used, split the prefetching code up into separate functions to prepare for a rewrite.

* Reworked prefetching to ensure we have a buffer of pages around ourselves. It is not fully tested, but working much better than previous implementation. Will be enhanced with DOM Pruning.

* Cleaned up some old cruft from the backend code

* Cleaned up the webtoon page change handler to use setPageNum, which will handle the correct prefetching of next/prev chapter

* More cleanup around the codebase

* Refactored the code to use a map to keep track of what is loaded or not, which works better than max/min in cases where you jump to a page that doesn't have anything preloaded and loads images out of order

* Fixed a bad placement of code for when you are unauthenticated, the code will now redirect to the original location you requested before you had to login.

* Some cleanup. Fixed the scrolling issue with prev page, spec seems to not work on intersection observer. using 0.01 instead of 0.0.

UI/Web/src/app/_guards/auth.guard.ts

@@ -10,6 +10,7 @@ import { AccountService } from '../_services/account.service';
   providedIn: 'root'
 })
 export class AuthGuard implements CanActivate {
+  public urlKey: string = 'kavita--auth-intersection-url';
   constructor(private accountService: AccountService, private router: Router, private toastr: ToastrService) {}
 
   canActivate(): Observable<boolean> {
@@ -19,6 +20,7 @@ export class AuthGuard implements CanActivate {
           return true;
         }
         this.toastr.error('You are not authorized to view this page.');
+        localStorage.setItem(this.urlKey, window.location.pathname);
         this.router.navigateByUrl('/home');
         return false;
       })