Move CI to GitHub Actions

[36degrees]

What

Move GOV.UK Frontend's CI from Travis to GitHub Actions

Why

Travis' changes to our plan means that builds of GOV.UK Frontend are getting stuck in a queue, which means they can take around 30 minutes to run.

Given that the direction of travel for the organisation seems to be towards GitHub Actions, and to be consistent with the approach we're taking with the Prototype Kit, move to GitHub Actions.

Who needs to know about this

Developers

Further detail

The current Travis config defines four jobs:

• Run the test suite
• Check that the Sass compiles using LibSass v3.3.0
• Check that the Sass compiles using Dart Sass v1.0.0
• Check that the Sass compiles using Ruby Sass v3.4.0

Equivalent jobs need to be run using GitHub Actions.

Done when

• [x] Existing open PRs merged / closed where possible to avoid needing to rebase
• [x] GitHub Actions config added
• [x] Travis config removed
• [x] Required status checks updated
• [x] Documentation updated
  • [x] Testing

[36degrees]

This is currently blocked because we can't currently update the required status checks in the GitHub settings – we get a 500 server error whenever we try and mark the new status checks as required.

I've been in touch with GitHub support and they're investigating – @vanitabarrett and @hannalaakso should now be CC'ed in the support thread.