Start to use msw to mock server requests.

So far have had to stop jsdom declaring that tests are running in a browser:

mswjs/msw#1786

Fiddle with whatwg-fetch polyfill:

reduxjs/redux-toolkit#1271

Still more things to fix, some presumably a consequence of first issue.

src/frontend/jest.config.js

@@ -6,6 +6,11 @@ export default {
     '\\.(css|less)$': 'identity-obj-proxy',
   },
   setupFilesAfterEnv: [
+    "@testing-library/jest-dom",
     "<rootDir>/setup-jest.ts"
-  ]
+  ],
+  testEnvironmentOptions: {
+    // https://github.com/mswjs/msw/issues/1786
+    customExportConditions: [''],
+  }
 };