This introduces Storybook into the Review Board tree. Storybook makes it

easy to test and play with UI components in isolation, providing an

interface for manipulating a component's state and seeing how it reacts.
Storybook is fairly self-contained. It has its own configuration files

in .storybook, and we'll define story files within docs/storybook.

We already use it for Ink, and in time we'll be able to publish

comprehensive Ink, Djblets, and Review Board component libraries as part

of our documentation.
We do need to tell it about all our JavaScript bundles and CSS that we

care about, somewhat duplicating the work that both the unit test suite

and the staticbundles.py have to do. Most of this will be able to go

away as we convert more of the codebase over to ESM-based bundles.
As a start, this contains one component for DiffComplexityIconView.

This was the inspiration for introducing Storybook, since I lacked a

great way of thoroughly testing this while updating it for dark mode

support.
Going forward, we should strive to introduce Story files for our

components as we do work on them, though this will very much be an

incremental process.

Ran storybook (npm run storybook), saw the Diff Complexity Icon

story, and played with values.