[MM-59552] Remove unsafe-inline from internal CSP, replace with nonce and rework some dynamic styling (#3120)

* Remove unsafe-inline

* Fix dynamic dark mode loading

* Include nonce generator for CSP for styles

* Add nonce provider for react-select

* Fix test

src/main/views/downloadsDropdownView.ts

@@ -66,7 +66,6 @@ export class DownloadsDropdownView {
         }});
 
         this.view.webContents.loadURL('mattermost-desktop://renderer/downloadsDropdown.html');
-        this.view.webContents.session.webRequest.onHeadersReceived(downloadsManager.webRequestOnHeadersReceivedHandler);
         MainWindow.get()?.addBrowserView(this.view);
     };