Add "Delete source branch when merge request is accepted" option in PRs for Gitlab/Gitlab selfhosted

We use Gitlab self-hosted and use the Pull Request feature.
The problem is, we need to select "Delete source branch when merge request is accepted." as a merge option.
Currently, we need to create a PR and always switch to GitLab for setting this setting

- Push feature branch to remote branch
- Drag & drop feature branch onto develop and select create pull-request
- Be able to set bit Delete source branch when the merge request is accepted.