New check: LICENSE doesn't contain variables

[laumann]

To check for the new QA policy.

The only exception [...] is the use of LICENSE itself, i.e. appending is allowed

[radhermit]

This should be a one-line change (not including tests)... but I guess from reading the policy it'll require exempting ${LICENSE} so it should be done in its own method (see MetadataVarCheck and ReferenceInMetadataVar).

[laumann]

Thanks for the tips @radhermit I'll see about implementing this soon.