Run swh-graph with gunicorn to support multiple/parallel requests

Currently it's ran on granet with the CLI: /opt/swhgraph_venv/bin/python3 /opt/swhgraph_venv/bin/swh graph rpc-serve -g /dev/shm/swh-graph/default/graph

This means it uses a single process, which is currently the bottleneck when processing a dozen queries in parallel.

The app path is a bit different from other packages, it's swh.graph.server.app:make_app_from_configfile()