Convert hooks server to OAuth GitHub App

[danuker]

We might want to convert the current GitHub hooks server to a GitHub App in order to:

• not require a personal token
• set up permissions for our hook server via the GitHub UI
• gain some knowledge about OAuth apps (secondary goal).
• maybe improve stability (since 50% of issues from Investigate hook failures #54 were due to Azure, not the script). But this is also addressable in Convert hooks server to PythonAnywhere? #56. That and this ticket might not be mutually exclusive.

In the future we will need to add support for OAuth in SFTPPlus server itself.

[adiroiban]

The main reason to add a GitHub App is to not require a personal token and to set up permissions for our hook server via the GitHub UI :)

For example, we now use the chevah-robot personal API key which has no access to non Chevah Org repos

[danuker]

Had to redeploy for updating the token. I think this ticket should no longer be low priority, but medium.

[adiroiban]

well... top priority is getting our code base to py3 :) everything else is low for now :(