Automated tests for Github user validation

[brockfanning]

Building on the CircleCI integration, it should be possible to implement some approximation of a simple user permission system, where the Github user names of contributors are validated against a curated list. That list would be public, since it would be part of this repository, but it would only contain Github usernames. I'll submit a proof-of-concept for review.

[brockfanning]

@Kali2017SDG @philipashlock
The proof-of-concept is available as a pull request: #898
The validation is only required when Github users want to change files in the data/ or _indicators folders. There is a short list of administrators who are exempt from the validation. Other users need to have allowed indicators listed. Wildcards can be used to simplify the lists. For example, instead of listing all the indicators under Goal 4, it could just be: 4-*-*.