Move Auspice dep from the server code to the auspice-client customizations

[tsibley]

See commit messages.

Related issue(s)

https://github.com/nextstrain/auspice/pull/1661

Testing

• [x] Build works locally
• [x] Checks pass

[tsibley]

So my first fix worked in CI… but failed locally when I tested it. That's because I tested it by doing:

rm -rf node_modules/ auspice-client/node_modules/ static-site/node_modules/
./build.sh

which failed when the Gatsby build couldn't find the iso-639-1 package. That's because it was still specified in the top-level deps instead of the auspice-client deps, and I'd removed the former. It worked in CI because npm ci was run before ./build.sh.

I fixed it again by moving that dep into auspice-client, which works locally with the above test.

As long as CI passes now, I'll merge.