Improve RDF linking and curations

[cthoyt]

motivated by improving the open educational resource collection:

1. This adds for SKOS and RDF downloads the ability to specify some extra metadata about their types
2. This adds a few new prefix curations (eli, erudite, openeduhub.context, lrmi vocabularies)
3. Makes a few minor curation fixes in existing records
4. Adds additional functions for working with collections