Replace github authentication workflow

[kcranston]

The current github authentication uses a username + password authentication to get a personal access token and then uses the token for all future API access. The ability to authenticate to the API using username + password is being deprecated, see https://developer.github.com/changes/2020-02-14-deprecating-password-auth/.

Proposed new workflow:

• create a GitHub App, abc-classroom-bot
• install the app on the organizations we need to access (earth-analytics-edu for testing and earthlab-education for production)
• authenticate individual abc-classroom users with Device Flow, directing them to a browser to enter a temporary code, which then allows us to get an access token
• use that access token for requests to the GitHub API

An initial proof-of-concept exists in the githubapp dir on the github-app branch of my fork.

[kcranston]

Implementation detail: planning to simply use requests and call the API directly rather than a using wrapper library for the GitHub API. Originally, we had planned to replace github3 with pygithub (issue #72 ), but neither library has support for the authentication steps and we only need support for two API calls:

• checking that a remote repo exists
• creating a new repo

[lwasser]

this has been completed!