Update GitHub gateway to new GitHub app

[radu-matei]

This issue tracks the progress towards migrating to the new Brigade GitHub App:

• Disable GH gateway by default
• Add GitHub App support to the Github Gateway
• Add check_suite support to the Github Gateway
• Delete the github.com/Azure/brigade-github-app (that will be replaced by this) edit (vdice): we've decided to keep this gateway in the form of a separate repo and chart

related: #407, #225, #318

[vdice]

I started circling around this today, re-familiarizing myself with brigade-github-app and what it would look like to swap out Brigade's current/classic github gateway with the former.

However, before getting into the details of what this may look like -- though, I will mention, it might just look like removing brigade's current gh-related files in pkg/webhook and swapping in those from brigade-github-app, as well as updating brigade-github-gateway/cmd similarly) -- I wanted to propse the following:

If the intention is to disable the GH gateway by default, as is the first bullet above, why not continue to host this (perhaps updated) gateway in Azure/brigade-github-app as well as continue to keep its separate Helm chart as exists today and just remove all GH gateway code from this repo itself? Then, enabling becomes a matter of flipping a chart flag and thus including the brigade-github-app sub-chart into the release. This would contribute to a more modular/composable Brigade setup, which seems to be a preferred way forward.

Thoughts?

[radu-matei]

Sure, that makes sense - however, would it make sense to migrate it over to brigadecore?

[vdice]

Yes, definitely -- assuming here all Brigade-related repos would migrate to brigadecore.

[vdice]

Pull requests are up with strategy mentioned above (remove older github gateway code/chart resources from Brigade in favor of opt-in approach to conditionally use brigade-github-app code/chart):

#797
Azure/brigade-charts#22

PTAL @radu-matei @adamreese @technosophos

[vdice]

Both aforementioned PRs merged; closing.