Stable branch MR helper

What does this MR do and why?

Adds a helper for identifying if an MR is targeting a stable branch not on the security repository. This is useful for identifying backport MRs for patch releases.

This helper will be used to enable and disable certain Danger rules that are specific to stable branches. For example, since backports have already been merged into the default branch, we do not need to display the size check warning for backports targeting older versions, so in this MR, we skip it for stable branch MRs.

Related to gitlab-com/gl-infra/delivery#2950 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.