Post to slack whenever someone manually merges something into kubernetes/kubernetes

[fejta]

I like a prow plugin that allows us to specify:

• Users allowed to merge code (k8s-merge-bot in our case)
• Slack channels to broadcast (sig-contribex and kubernetes-dev)

Then whenever a user who is not on the whitelist merges something into the repo we send a slack message to those channels.

Example of a: Submit queue merge: https://github.com/kubernetes/kubernetes/pull/45773#event-1118790064 Merge by a human: https://github.com/kubernetes/kubernetes/pull/46813#event-1116483350

In other words when Tim merged 46813 we want to see a note in kubernetes-dev and sig-contribex that says something to the effect of Warning: @thockin manually merged https://github.com/kubernetes/kubernetes/pull/46813

[fejta]

/area prow

[brahmaroutu]

/assign

[spiffxp]

in an effort to herd submit-queue related discussion to one place, I think #sig-release would also probably be a good channel, at least during code freeze if nothing else

[ixdy]

it'd also be good if it could alert if someone pushes commits into the upstream branches directly (without going through a PR at all), though maybe not alert on the release manager doing version/tag pushes.