Ensure that UCSC's browser branches are pushed to GitHub

[hannes-ucsc]

Despite the Azul operator manual mentioning it, operators are prone to forgetting that they need to push each ucsc/*/* branch to GitHub before pushing it to a GitLab instance (example). We need the branches on GitHub so that we have a public record of what was deployed at what point in time. The GitLab instances are not accessible to the public and compliance requires us to maintain a backup of important records such as those.

For Azul this is enforced by GitLab publishing status checks to GitHub. GitHub's status check API returns an error if the status check refers to a commit that's not on GitHub. That error breaks the build early on.

We should do the same for the Data Browser.