UI: Instant-on manager

[ghengeveld]

Issue: #12702

What I did

This implements an instant-on manager UI by bundling a prebuilt manager with the Storybook package, as well as caching a locally built manager. The cache gets busted whenever a change is made to manager.js or the webpack config (typically via main.js). There's also a --no-manager-cache flag which disables caching and ignores the prebuilt manager. The prebuilt manager is also used when building a static Storybook, assuming all criteria are met.

▶️ Watch the demo

How to test

• Run start-storybook against a vanilla, newly installed Storybook which only has addon-links and addon-essentials installed. It will say "Using prebuilt manager" and skip building the manager (see the startup message, it won't mention the manager build time).
• Run build-storybook against the same Storybook, and notice it will use the prebuilt manager here as well.
• Run start-storybook against any other Storybook. The first time it will build the manager as normal. Then kill and run start-storybook again. Now it will say "Using cached manager" and work the same as above.

The --no-manager-cache flag should be added to the docs.

To do

• Refactor the dev server for readability / maintainability
• Start the browser as soon as the manager is ready
• Show an appealing and useful UI as the preview builds
• Use prebuilt when creating a static build
• Setup our publish pipeline to prebuild the manager before publishing

[shilman]

This is looking great. How do we publish this as part of the release? Is that a separate PR or an addition to this one?

[ghengeveld]

🚀