Use the same OPAM new package table generation system as opam2web

Fixes #253. Fixes #326. Fixes #412.

Not the prettiest implementation or least redundant but it gets the job done. The server will need to have opam installed and periodically run

```bash
opam update
script/generate_opam_update_list
make
```

or something similar. I don't really know what the present update script looks like because it's not in this repository for some reason.

Also, I didn't know whether to list more transitive dependencies in the README or if just opam2web >= 1.3.1 was enough.

It's probably wise to play around with this a little locally and then on a test server before actually deploying but I have no idea what the deploy procedures look like. I hope this works for you. I'll be happy to support this feature now and in the future.