Import more license identifiers

This site has a very comprehensive list of all open licenses:
https://spdx.github.io/spdx-spec/appendix-I-SPDX-license-list/#i1-licenses-with-short-identifiers

could be useful to add to https://github.com/learningequality/le-utils/blob/master/le_utils/resources/licenselookup.json to allow more precise license information (and versionning on the CC licenses)
