Replace license URLs with SPDX identifiers
See: https://spdx.org/sites/cpstandard/files/pages/files/using_spdx_license_list_short_identifiers.pdf
Currently we are just referencing URL to upstream's license. It would be way more descriptive to include an identifier for the actual license instead, and will also help deduplicate non-free licensing logic (see DuckDuckGo, iOS Onion Browser entries).