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/util/metadata.ts

@@ -7,7 +7,6 @@
  */
 
 import { mutate } from 'swr';
-import client from 'util/client';
 import {
     AllowedMetadataValueTypes,
     AppMetadataKeys,
@@ -20,6 +19,7 @@ import {
     IMetadataMigration,
     MetadataKeyValuePair,
 } from 'typings';
+import requestManager from 'lib/RequestManager';
 
 const APP_METADATA_KEY_PREFIX = 'webUI_';
 
@@ -314,7 +314,7 @@ export const requestUpdateMetadataValue = async (
         [metadataKey]: valueAsString,
     };
 
-    await client.patch(url, formData);
+    await requestManager.getClient().patch(url, formData);
     await mutate(
         urlToMutate,
         { ...metadataHolder, ...wrapMetadataWithMetaKey(wrapWithMetaKey, mutatedMetadata) },