preparation for msw 2.x migration

[deer-wmde]

Basic preparations for reimplementation of mock service handlers.

Related (basically the same work as): #1114

• ran 'npx msw init public --save'
• start mock service with no handlers
• old handlers kept for now as reference in old.default_handlers.js
• commented import line left as a hint for starting with implementations; commented to make linter happy

https://phabricator.wikimedia.org/T436358

[github-actions]

Deployment previews on netlify for branch refs/pull/1180/merge will be at the following locations (when build is done):

• UI with mock store: https://17099f8944-mock--wbstack-ui.netlify.app/
• UI pointing to prod: https://17099f8944-prod--wbstack-ui.netlify.app/