Investigate using GitHub Actions (vs Travis and/or BuildBot)

[steven-johnson]

Per https://github.blog/2019-08-08-github-actions-now-supports-ci-cd, this looks like (at least) a replacement for Travis; it looks like it might also allow for using self-hosted builders, in which case we might be able to also replace BuildBot usage and keep our custom buildbot machines for now. Someone should investigate.

[alexreinking]

@steven-johnson -- any updates on this?

Some relevant PRs:

#4792
#4767
#4680
#4657
#4667
#4618

Some relevant issues

#4752
#4754
#4753

[steven-johnson]

Yeah, this should still stay open. Unfortunately GHA still doesn't have the features needed to make it a good replacement for our Buildbots; in particular, there still isn't a good way to throttle redundant jobs (e.g., if a PR is opened and then one or two oopsie-fixes are pushed, there isn't a good way to collapse those into a single build or to kill the redundant ones).