fix: define and/or use an existing type for GitHubEventAction

[mattxwang]

Currently, GitHubEventAction uses an any prop for the payload:

opensource/components/GitHubEventAction.tsx

Lines 5 to 8 in 87b2993

	interface GitHubEventActionProps {
	type: string,
	payload: any,
	}

This shouldn't have to be the case, since octokit defines types for all of its request/responses, including the payload of an API hit. The reason I haven't done this yet? It's hard to find!

So, let's:

1. find the type, and/or create a TS Union of the relevant types
2. simplify the existing code in GitHubEventAction to use the new type, and avoid unnecessary null / ? checks

[reginawang3495]

@mattxwang oops there is no accurate schema octokit/webhooks#602 (comment) 😓. Webhooks is pretty accurate, so I think we can use that for now esp since Nick has been doing some really great work with it, and slight type issues aren't the end of the world.

We can update this once they have an events types up + DM me if you have other thoughts?