Feature/streamline backend requests (#297)

* Introduce "RequestManager"

* Use "RequestManager" - Simple replacements

 - Get rid of all "util/client" imports
 - replace old requests with new "RequestManager"
 - remove "fetcher" from global SWR config
     instead of using the SWR hooks by themselves, the "requestManager" is supposed to be used

* Use "RequestManager" - Do requests via SWR hooks

Use SWR hooks at places where it's easily usable

* Prevent trailing slashes in the "baseUrl"

src/typings.ts

@@ -379,3 +379,20 @@ export interface BatchChaptersChange {
     isBookmarked?: boolean;
     lastPageRead?: number;
 }
+
+export type UpdateCheck = {
+    channel: 'Stable' | 'Preview';
+    tag: string;
+    url: string;
+};
+
+export type SourceSearchResult = {
+    mangaList: IManga[];
+    hasNextPage: boolean;
+};
+
+export type BackupValidationResult = {
+    missingSources: string[];
+    missingTrackers: string[];
+    mangasMissingSources: string[];
+};