Improve and fix release process

We currently rely on a script for the release process with a long list of steps.
It can and has failed at any one of these steps and there is no way to automatically recover if a failure happens.

The script is more or less our implementation of the [Git flow workflow](https://www.atlassian.com/git/tutorials/comparing-workflows/gitflow-workflow). We should instead use the [git-flow](https://github.com/nvie/gitflow) git extensions