Change license acceptance criteria

[sellout]

• allow license exceptions by default – specific ones can be added to a reject list (fixes License check rejects FOSS license (in SPDX format) #1440)
• allow + licenses
• report the portion of the license that is unacceptable in the error message (improves Better error message on license check #710, but could be more explicit about why licenses are rejected)
• extract the constant allow/reject lists from the evaluation function

[sellout]

I’m happy to change any specific parts of this – e.g., change the license exceptions to an accept list instead of a reject list (as long as I get to add Universal-FOSS-exception-1.0 to the accept list) or implement more specific rejection of + or other deprecated licenses.