Set up / update EML validator web servlet for 2.2.0

[amoeba]

During a discussion with @mobb on NCEAS/#eml today, she expressed that having the old web validator back for EML 2.2.0 would be super handy for the community. There are other ways to validate EML records, including the R package and checking out this repo and running the validate script in ./bin but those aren't nearly as accessible as a simple web page.

I think we mostly all agree but need to pencil out the details:

• Yay? Nay?

• Location: Historically is/was https://knb.ecoinformatics.org/emlparser/. Should we just plop the new one in place here?

  If I was setting this up from scratch, I'd probably lobby for the location to be eml.ecoinformatics.org/validate but that looks hard to do with our Netlify deployment unless I'm mistaken.

• Link this into documentation?

[mbjones]

We can;t deploy under Netlify, which is why I skipped it in the first place.

Today I updated https://knb.ecoinformatics.org/emlparser/ to use the new EML 2.2.0 build, so it should now work against all versions of EML.

[amoeba]

Thanks, @mbjones. I think it'd be good to include a link in the readme at the top along with the other links we provide. Sound good?