feat(config): Automatically detect GitHub config when missing

[BYK]

We don't need people to explicitly state github global configuration
as it can be detected from git itself. Since we rely on the presence
of a git repo in publish and prepare we can safely rely on this
too and remove another piece of redundant configuration.

Most of the changes in this patch are related to the conversion of getGlobalGithubConfig from a sync function to an async function. There were also a few places which relied on config.github directly which is also removed now.

Ideally we'd abstract the raw config object in the future and provide a global, normalized configuration to all downstream dependent code.

This patch also adds tests around this new functionality.

[BYK]

I will add the tests for this new functionality separately as they cause some very hard to debug import errors. I'm guessing it has something to do with Jest and mocking but I've already spent multiple full days on debugging without any luck and now this PR blocks #209 (due to its github config refactor).

[chadwhitacre]

Can we default this to null/undefined to avoid having to modify every target, since most targets don't need this?

[BYK]

I'm not a fan of that as then we need to do non-null checks down the line. This will especially be pronounced in publish where we get all the targets without knowing the exact target type (as it is determined at runtime).

[chadwhitacre]

Do we? Why would we need to check for null if the only targets that use githubRepo are the ones that will actually have it in the constructor?

[BYK]

Even if they have it in the constructor, the type system will force us to make sure it is not null. We may be able to overcome this so I'll look into that later.