Investigate bundling Flagsmith into a single executable

e.g. using [pyinstaller](https://pyinstaller.org/en/stable/index.html)

The requirements are to be able to run flagsmith as an executable in a linux environment, without having to use Docker.

In order to do this, we'll need to create a script to wrap gunicorn and run Flagsmith, then use pyinstaller to bundle the application. There's a gist [here](https://gist.github.com/matthewelwell/2ea8281924b8bca5233b45baa30fe5f6) (mostly taken from [here](https://stackoverflow.com/questions/70396641/how-to-run-gunicorn-inside-python-not-as-a-command-line)) that mostly works for actually getting the API running using gunicorn in a python script - we'd just need to extend it to take command line arguments to configure gunicorn correctly (hopefully we could make this pretty transparent).