Add contribution_utils to license_utils for more complete unit testing

[netsettler]

This is a layered PR adding changes to allow checking of contributors.

• See in particular the change to qa_checkers.py adding ContributionsChecker.validate(), which will check that a CONTRIBUTORS.json file exists and summarizes who's contributed to the repo. This will allow us to make sure contributions come from employees, or to check that other contributions are made by people agreeing to our contribution policy.

[coveralls]

Pull Request Test Coverage Report for Build 5694652740

• 631 of 653 (96.63%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.3%) to 77.248%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
dcicutils/contribution_utils.py	316	338	93.49%
<!--	Total:	631	653	96.63%	-->

Totals
Change from base Build 5590491337:	1.3%
Covered Lines:	8084
Relevant Lines:	10465

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💛 - Coveralls