Basic GitHub Action for release

[rnorth]

Refs #1835

[bsideup]

nice! 😍

[bsideup]

@rnorth since we added the ./gradlew check to the release, we may have a problem of timeouts (will test everything, including the modules)
Perhaps we should also migrate other test stages we had in Travis and depend on them in the release stage, so that we run core, modules and other stages in parallel?

[rnorth]

As discussed briefly on Slack, we'll make this release step verify that checks are succeeding for the branch to be published.

I'll code up a GitHub Action to do that separately.

For this PR, I've just removed the explicit check step: we can easily manually verify that master is green before doing a release, until there's an automated GitHub Action to act as a guard.

[bsideup]

@rnorth I was wondering if a simple shell script is sufficient for it, since we don't need an authentication to access the commit's status, and can simply curl it and assert the result (this part may not be trivial, curious if GHA have jq installed)

[rnorth]

Helpfully, jq is available, so will do that!

[bsideup]

One last thing - I just spotted that the file name (gradle-release.yml) and the stage's name are a bit too specific.

Although I don't mind being specific about the step itself (e.g. "Publish to Bintray"), I am not sure we need "Gradle" in the name :)

[rnorth]

Ah OK, I don't really mind much either way, so can change!

[bsideup]

Don't mind as it is, although making it less specific about the step would be a nice improvement :)