Create GitLab merge request in JIRA issue in development panel

Follow up to https://gitlab.com/gitlab-org/gitlab-ee/issues/2647.

Create a GitLab merge request in the JIRA issue's development panel. GitLab automatically creates a branch. Afterward, both the branch and merge request are viewable from the dev panel.

Interested Customers:

• https://gitlab.my.salesforce.com/00161000010e9dZ
• https://gitlab.my.salesforce.com/0016100001HGPYw
• https://gitlab.my.salesforce.com/0016100001TC9JQ