IIIF validation

[camdendotlol]

Summary

• adds a bunch of checking for null values and array length throughout the iiif.js script to make sure we're getting the data we need to properly generate the TEI file
  • some of these checks address potential crashes (e.g. accessing the first element of an array that might be empty) while others focus more on user error (e.g. undefined labels and empty item arrays)
• adds newlines and indentation between each surface element so they're no longer all printed on one line
• updates the TEI template used by the iiif script to wrap the contents in a div

Notes

We have to be really careful here because there's so much variation in how IIIF manifests can look - we don't want to error out on a valid manifest. So this PR takes a pretty minimalist approach to these checks and doesn't attempt to fully validate the manifest against the IIIF standard. I've tested with a wide range of manifests with both versions of the Presentation API and they all worked, but it might be worth testing more widely before a production release.