GitHub App instead of GitHub oAuth app

[amirbey]

User Story

Status repository updates are sent to the repository(PR) via a GitHub app instead of using a user's token

Background (Optional)

When commits are made to the repository by a non Pages user, a status update is sent to the repository using a Pages GitHub App (token?).

Acceptance Criteria

• build status are sent using Pages/Federalist GitHub App
• Change made live via deploy of Federalist

After evaluating, edit this part:

Level of effort - Medium

Implementation outline (if higher than "low" effort):

• create a GitHub app
• use the GitHub app (token?) to send status updates instead of user tokens
• App has to be accepted by organization admin -- update docs Permissions section

[amirbey]

@18F/federalist-admins to review with @pburkholder for prioritization