Restore GPL copyright header configuration

This partially reverts changes (accidentally) made in fb89f08, restoring
IDEA copyright header configuration files back to their original state
as committed in d5e7b6c.
Add missing GPL headers

Per 8746ab7, IDEA's copyright configuration had been wiped out for some
time. Now that it is back in place, this change is the result of a
global "Update copyright..." action (with a bit of pruning afterward).