Validate XML resources when uploading.

[benanhalt]

[maxpatiiuk]

Added client-side live validation for XML and JSON resource files.

[benanhalt]

Nice. Eventually I would like to validate the schema of the resources as well, where possible. I started defining the schema for the Specify forms here: https://github.com/specify/viewset-schema

[maxpatiiuk]

I see. I opened a separate issue for that.