Move osrd-ui into main osrd repository

[emersion]

Description and goal

A pain point when working on osrd-ui is that it's tricky to work on osrd-ui and test the changes in OSRD in a real-world scenario instead of a storybook. Additionally, osrd-ui changes are costly: each time we want to update osrd-ui (e.g. for a bugfix or small enhancement) we need to create a new osrd-ui release and then bump the osrd-ui dependency in OSRD's package.json.

We could move osrd-ui into a subdirectory of the main OSRD repository. osrd-ui's NPM packages would continue to work as usual. OSRD's front could pull osrd-ui from a local directory instead of the NPM registry.

Possible roadblocks:

• How do we integrate osrd-ui's multi-package/lerna setup into OSRD's front build process ?
• Do we want to retain git history? (It's possible)
• yarn vs npm?
• Release process? Right now we use git tags, GitHub releases and a GitHub actions workflow.

Acceptance criteria

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