Add Github Actions CI

[sohkai]

See results in the Actions tab: https://github.com/aragon/aragon-court/actions.

Commit by commit:

1. The naive setup takes quite a lot of time, and allows for no test parallelization. You will likely be waiting a long time to get results
2. An attempt at caching dependencies via upload / download fails spectacularly. It takes forever to upload the node_modules; no go
3. The naive parallelization is closest to what is available on Travis, but still looks quite slow in comparison.

However, it looks like the Github Actions runner is still more starved for resources than other equivalent CI platforms, and is unreliable due to how many files ganache needs to touch (EMFILE errors seem to be common). I've sent a help ticket for more guidance on this, so let's see if this is something we can change.

[sohkai]

Going to close this for now; it does not look like Github Actions CI is reliable enough for large contract testing at the moment.