Add fastapi app

[ricardogsilva]

This PR adds a FastAPI app to serve as the entrypoint for the backend.

Fixes #36

It is composed of one main app which then mounts three sub-apps. These are accessible at the following prefixes:

• /legacy - This is where the original django web app is mounted. It is ran with the FastAPI WSGIMiddleware and the implementation is based on the FastAPI docs, but with some considerable modifications in order to adapt to django. The django app's admin pages static files are also served by FastAPI using an additional mount
• /v1 - This contains a reimplementation of the legacy app's API, done using FastAPI and pydantic. It thus provides an openapi document and some dev docs, which document how it can be used. Not all path operations are reimplemented yet, but at least the API documentation should be
• /v2 - This contains an initial draft of what a more flexible API could look like. There aren't many endpoints in it yet, but it notably contains a proxy to THREDDS which is able to serve WMS requests.

Also included:

• Unified settings management, whereby the settings for the django application are provided by pydantic
• CLI commands to:
  • Launch the server using uvicorn
  • Run django-admin commands
• Refactor of the Dockerfile in order to cope with the new FastAPI application
• Refactor of the docker compose file
• Refactor of the tests in order to cope with the new FastAPI application