Add Gitpod support and guide

Changes

I've added and configured .gitpod.yml in a way that when you start a new workspace, it runs a Funkwhale instance with docker that is configured according to the CONTRIBUTING.rst. You can test the behavior navigating to https://gitpod.io/#https://dev.funkwhale.audio/funkwhale/funkwhale/-/tree/wvffle/gitpod

I've also added a section about Gitpod to the CONTRIBUTING.rst

I removed some unused parts of nginx configuration that I guess are remaining from the webpack era. (Haven't found any occurances of sockjs and front-server in current node_modules using grep)

Additional context

I downgraded the alpine version to 3.14 for python-ldap to build correctly and to be able to create a Gitpod prebuild.

Fullstack instance asks whether the user wants to open the preview or not and frontend only server (which only starts if you open a workspace in /workspace/funkwhale/front by hand due to the vite extension for vscode) opens a preview automatically. My reasoning for that behavior is that you may not be interested in the frontend when you develop backend and you may not be interested in backend while developing frontend. The vite vscode plugin also integrates better with vscode than the vite server served by docker.