Handle external issues in IssueReferenceFilter

What does this MR do?

This MR fixes IssueReferenceFilter so it properly renders issue references such as #1 as external issue links whenever the corresponding project uses an external issue tracker.

Are there points in the code the reviewer needs to double check?

Currently IssueReferenceFilter duplicates a bit of code used to determine the title for a link. I can't really think of a way to share this logic with ExternalIssueReferenceFilter in a way that isn't messy.

Why was this MR needed?

Rendering issue references such as #1 was broken for projects using an external issues tracker.

What are the relevant issue numbers?

gitlab-org/gitlab-ce#19036,

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together