Do not create stable branches as part of monthly release issue creation

This MR removes stable branch creation from the preparation process. Instead, it provides a rake task to do this separately, on-demand via a chatops command (added in gitlab-com/chatops!72 (merged))

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.

Discussion in slack

Marin: Can you remove creating a stable branch from the monthly preparation task? It should not create those branches at the moment we create an issue.

Balu: I can do that, but when should it create stable branch instead?

Marin: When we tell it to? 😛 Right now issuing a chatops command to prepare a release the task will: Create an issue, create stable branches if they don't exist. Stable branch should be created when we are ready to create a stable branch and not at the moment we are ready to start working on a release.

Possible future iterations

• Instead of passing the auto-deploy branch to create stable branches from, find a way to idempotently detect which is the auto-deply branch currently deployed in gitlab.com and create stable branch from that.