Deploy pull request previews to DigitalOcean spaces

[MattIPv4]

With the current deployment system, to test a full build of the tools in a pull request, we must pull the branch down, run the build scripts locally and then spool up a web server in the dist directory to test that the pull request builds as expected.

It'd be great if we can add a secondary deployment to the Travis config that only applies to pull requests and deploys the latest commit in that PR to DigitalOcean spaces and reports back the URL so that we can easily review and confirm that the pull request builds as expected in-browser.

[MattIPv4]

It looks like we've just unlocked GitHub Actions on the org - might be worth, as part of this, migrating our entire testing & build process to Actions, though I'm sure if there are decent providers for deploying to gh-pages & DO Spaces in Actions yet.