Integrate Sentry and GitHub

Summary:
A custom GitHub application has been created in the Nasqueron account,
and credentials published to Vault under apps/sentry/github key.

The keys under apps/sentry/github match the expected settings names,
that allows the use of the for loop instead of querying the 6 of them.

Those credentials are in apps/ and not ops/secrets/ as they don't need
to be deployed by Salt, they can directly been fetched by Sentry config.
As such, the updated Vault policy is the one used by Sentry config.

Reference: https://develop.sentry.dev/integrations/github/

Ref T1806

Test Plan: Deploy on docker-002 for sentry_web

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1806

Differential Revision: https://devcentral.nasqueron.org/D2920