Allow source branch deletion for closed merge requests

Problem to solve

When a developer closes a merge request, it is not possible to easily delete the related branch.

This is a flow that could be very common. Users close merge requests and they don't need the proposed changes anymore.

When users merge a request, there is an option to delete the branch in case it was not already done automatically. We need something similar also for closed merge requests.

Target audience

• Delaney, Development Team Lead, https://design.gitlab.com/research/personas#persona-delaney

• Sasha, Software Developer, https://design.gitlab.com/research/personas#persona-sasha

Further details

Having this as the default option is not optimal because there are legit cases where the code is still useful even after the merge request has been closed. Users should explicitly delete the branch.

Proposal

Add a button to delete the branch when a merge request is closed, similar to what we have for merged ones.

What does success look like, and how can we measure that?

Number of branches removed because of this feature.