Verify the existance of a commit via GitHub API and that the given commit is in the master/main branch

[potomak]

[bugant]

I think I'll this API https://developer.github.com/v3/repos/commits/#list-commits-on-a-repository

By default it will list all commits for the project's default branch. I think we can also filter commits by author (the user who submitted the resolution, but this would require changes to the contract). And also set a maximum time span to avoid incurring in rate limits due to requests that make us scan an entire big project commit history.