Move all the software license stuff to a central location.

This leaves an appropriatesummary on the website pages, committer's
guide and contributing pages, and augments license-guide with
information that was on those pages (though the license exceptions
moves to software-license).

It's mostly word motion with light edits.

Sponsored by: Netflix
Reviewed by: ziaee, adrian
Approved by: core (24 hour rule)
Differential Revision: https://reviews.freebsd.org/D57340