Navigate to external issue tracker page

• When an external issue tracker is activated, provide a link to the external issue tracker page from the GitLab navigation.
• Note that the internal GitLab issue tracker may or may not be activated. In the mockup below, it is.
• Per https://gitlab.com/gitlab-org/gitlab-ce/issues/37039, there should only be external issue tracker active. (That constraint may not be enforced in the system. So just pick one if that is the case, without any particular requirement.)
• Do not show how many issues there are.
• The link title is JIRA, Bugzilla, or Redmine.

The external trackers are:

• Bugzilla, e.g.: https://gitlab.com/gitlab-org/gitlab-ce/services/bugzilla/edit
• Custom Issue Tracker, e.g.: https://gitlab.com/gitlab-org/gitlab-ce/services/custom_issue_tracker/edit
• JIRA, e.g.: https://gitlab.com/gitlab-org/gitlab-ce/services/jira/edit
• Redmine, e.g.: https://gitlab.com/gitlab-org/gitlab-ce/services/redmine/edit

SVG: Icon for external issues