[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/app/initialize.test.js

@@ -64,6 +64,9 @@ jest.mock('electron', () => ({
     },
     session: {
         defaultSession: {
+            webRequest: {
+                onHeadersReceived: jest.fn(),
+            },
             setSpellCheckerDictionaryDownloadURL: jest.fn(),
             setPermissionRequestHandler: jest.fn(),
             on: jest.fn(),