CI job for licensing signoff

[victorb]

From @lgierth on August 6, 2018 22:53

From ipfs/infra#228

It would be useful to have a CI job listening for changes on the relevant repos, verifies the license signoff, and updates the PR's status through the github API.

In the end it'd be similar to gitcop's functionality, but closer to our needs.

Copied from original issue: ipfs-inactive/jenkins#121

[victorb]

From @mikeal on August 6, 2018 23:20

Related issue at ipfs/community#331

One thing I want to make sure we do here is make the instructions/errors new contributors see as nice as possible. Most of the people who violate these will be new contributors who just haven't learned our policies yet.