Add Storybook support for Element

To help the discovery and development of UI components we want to use [Storybook](https://storybook.js.org/). A developer would be able to create stories alongside components in `matrix-react-sdk` or `element-web` to preview certain components and understand how they behave when provided with a different set of properties

# Setup for element-web

The project is currently split across `element-web`, `matrix-react-sdk` and `matrix-js-sdk`.

All the information to compile the project, load the CSS are located in `element-web` whereas the components live in the React SDK.

Ideally we would want the storybook configuration to live in `element-web` so that it can piggy back our webpack configs. I have experimented with that setup and faced an issue that I raised on the main repo, storybookjs/storybook#15694

We can alternatively have the entire Storybook setup living in the React SDK. This means that we will need to mimic the webpack configuration that `element-web` has. This could lead to a disjointed experience when components are loaded in a Storybook context or inside `element-web`.
This problem could be mitigated by making those limitations clear to developers so that they make sure to test their code in both places