Use Danger to validate backport MR title

Context

We have a requirement for the title of backport MRs:

• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.

We have some requirements for backport MRs, some are in the backport MR template, and some are in the Danger check. It would be nice if we can enforce a rule for the title in the Danger check, so MR assignees cannot forget. Because the title will appear in the patch blog post, it is important to have it standardized and easy to read.

Exit Criteria

• Danger check for stable branch MRs includes a check if the title starts with "Backport of"