CE-2750: Create releases on merges to main

See https://gitlab.esss.lu.se/help/user/project/releases/release_cicd_examples.md#create-a-release-when-a-commit-is-merged-to-the-default-branch for the mechanism here. Note that if we have two separate merges to main without updating the version in package.json, then the second one will fail as this does tag the repository. This is probably fine.

Should we add a dependency one way or the other between release and deploy-package? I.e. should we first create a release, and then deploy that release?

To see how this works, see this test project: https://gitlab.esss.lu.se/simonrose/release