Move `random.js` from `src/sidebar/util` to `src/shared`

This file will be useful for components in `src/annotator` so it will be
better to move it to common path for both the sidebar and annotator.

src/sidebar/services/annotations.js

 /** @typedef {import('../../types/api').Annotation} Annotation */
 /** @typedef {import('../../types/annotator').AnnotationData} AnnotationData */
 
+import { generateHexString } from '../../shared/random';
 import * as metadata from '../helpers/annotation-metadata';
 import {
   defaultPermissions,
   privatePermissions,
   sharedPermissions,
 } from '../helpers/permissions';
-import { generateHexString } from '../util/random';
 
 /**
  * A service for creating, updating and persisting annotations both in the

src/sidebar/services/streamer.js

+import { generateHexString } from '../../shared/random';
 import warnOnce from '../../shared/warn-once';
-import { generateHexString } from '../util/random';
 import { Socket } from '../websocket';
 import { watch } from '../util/watch';
 

src/sidebar/services/toast-messenger.js

-import { generateHexString } from '../util/random';
+import { generateHexString } from '../../shared/random';
 
 // How long toast messages should be displayed before they are dismissed, in ms
 const MESSAGE_DISPLAY_TIME = 5000;

src/sidebar/util/oauth-client.js

+import { generateHexString } from '../../shared/random';
 import { fetchJSON } from './fetch';
-import { generateHexString } from './random';
 
 /**
  * An object holding the details of an access token from the tokenUrl endpoint.

src/sidebar/util/postmessage-json-rpc.js

-import { generateHexString } from './random';
+import { generateHexString } from '../../shared/random';
 
 /** Generate a random ID to associate RPC requests and responses. */
 function generateId() {

src/sidebar/util/test/oauth-client-test.js

@@ -214,7 +214,7 @@ describe('sidebar/util/oauth-client', () => {
       fakeWindow = new FakeWindow();
 
       $imports.$mock({
-        './random': {
+        '../../shared/random': {
           generateHexString: () => 'notrandom',
         },
       });