Update GitHub app auth flow, migrate docs to website

[alexwaibel]

Updated the GH app auth flow and bumped up the Octokit version.

I've also added some of the important config values to the Heroku manifest so it prompts users for them when they deploy:

This, combined with the updated documentation website (PR for those updates here) should help ease the confusion many new users have around installation.

[alexwaibel]

I need to investigate why this build is failing. The relevant docs changes are up on staticman.net though, so once this change is out hopefully the setup process will be more clear.

[eazydripz]

❤️