Create GitLab branch from Jira issue

Follow up to https://gitlab.com/gitlab-org/gitlab-ee/issues/2381.

Create a GitLab branch from the JIRA issue's sidebar. Subsequently, the branch is associated with the issue, per https://gitlab.com/gitlab-org/gitlab-ee/issues/2645.

The branch should be named after the JIRA issue title / user story key.

We can implement this in our GitLab for Jira app (Jira Connect Integration) in the Atlassian Marketplace. This would only be available for GitLab.com + Jira Cloud though.

As described in #2650 (comment 199989353):

If we want to support creating of branches or branch and MR just like in our issues we'd have to implement it using this API: https://developer.atlassian.com/cloud/jira/platform/issue-view-ui-locations/. This allows us to show arbitrary content in Jira's issue view.

We can do this with the same Atlassian Marketplace app that we're building so that once they install it, they get the dev panel integration and this create branch button.

I don't this is trivial though because we don't have a 1 to 1 mapping of Jira projects and GitLab projects. If we did, this could be a simple button that links to https://gitlab.com/namespace/project/-/branches/new which already exists.

So we might need to link it to a new page where the user can create a branch with a project selector with all the projects the user has access to or implement the project selector in the Jira UI (which is a bit harder IMO).

Design spec

new-branch-flow

👉 Figma link

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