Add pull mirror branch prefix

What does this MR do?

This MR allows specifying a branch prefix on pull mirroring, it will be useful on forked project to keep track of upstream branches on a dedicated branch namespace.

Moreover, it will help us moving security releases to gitlab.com implementing the following diagram

graph LR
  subgraph Canonical
    c-master(master)
    c-stable(12-3-stable)
    c-auto-deploy(12-3-auto-deploy-20190911)
    c-feature(feature/some-new-feature)
    c-bug(15015-some-bug-fix)
  end

  subgraph Security
    s-master(security/master)
    s-stable(security/12-3-stable)
    s-auto-deploy(security/12-3-auto-deploy-20190911)
  end

  c-master -.->|pull mirror| s-master
  c-stable -.->|pull mirror| s-stable
  c-auto-deploy -.->|pull mirror| s-auto-deploy

Related to gitlab-com/gl-infra/delivery#472 (closed)

The following feature is available under the :pull_mirror_branch_prefix feature flag

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team