Add chatops command to create stable branch for release

This MR adds a new chatops command to create stable branches in different projects from a specified source branch, so that stable branch creation can be decoupled from preparation for a release.

Context

Before auto-deploy process was in place, the preparation for a new release happened around 1st of each month. This preparation involved creation of an issue, a "pick into" label and stable branches for the release. Around 7th, the stable branches were updated with master at that time and after that they were updated only using a "pick into" label based automatic process.

However, with auto-deploy in place, stable branches are essentially same as master branch as latest as of 18th (technically, the last auto-deploy branch that was deployed in gitlab.com). This means it no longer makes sense to create the stable branches early on, when we start preparing for a release. Instead, we want a way to create stable branches on-demand, from a source branch we specify. This chatops command brings exactly that.

It is dependent on gitlab-org/release-tools!688 (merged)