Pathfinder iframe integration for testing on devsites

[bobular]

A pathfinder demo instance is running on port 3000 on cedar, via my userspace systemd config. See https://github.com/VEuPathDB/pathfinder/blob/main/README-podman-quadlets.md for details.

You can access it on http://localhost:3000 after running ssh -L 3000:localhost:3000 cedar locally to set up a tunnel.

This PR integrates the new feature into an iframe (possibly temporary just for further testing until tighter integration is engineered). The iframe is available at /a/app/pathfinder, but will require proxying before it will work.

Request: Apache proxy for Pathfinder app

The Pathfinder webapp runs as a separate Node process on port 3000 of the server. We need Apache to proxy a path to it so the iframe can load it same-origin rather than directly hitting the port.

Add the following to the Apache virtualhost config for the relevant sites on cedar:

ProxyPass /pathfinder-app http://localhost:3000
ProxyPassReverse /pathfinder-app http://localhost:3000

This must be placed before any catch-all or webapp handler rules so it takes priority.

Effect: Requests to https://plasmodb.org/pathfinder-app?... will be proxied transparently to http://localhost:3000?.... The React embedding page at /a/app/pathfinder is unaffected.

Required Apache modules: mod_proxy and mod_proxy_http (likely already enabled).