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

Added 3 commits:

• 30f7b906...d33991f8 - 2 commits from branch master
• 9efcda32 - Handle external issues in IssueReferenceFilter

Added 1 commit:

• a6b92951 - Handle external issues in IssueReferenceFilter

mentioned in issue #19201 (closed)

mentioned in issue gitlab-com/support-forum#575

mentioned in issue #19248 (closed)

Added 12 commits:

• a6b92951...bef4294c - 11 commits from branch master
• f2a988f8 - Handle external issues in IssueReferenceFilter

mentioned in issue #19036 (closed)

Added 1 commit:

• 33bda9c3 - Fix broken spec in git_push_service_spec by stubbing an external issue tracker

Added ~149423 label

Added 2 commits:

• a4ce2d12 - Handle external issues in IssueReferenceFilter
• 4fca633a - Fix broken spec in git_push_service_spec by stubbing an external issue tracker

mentioned in commit 6e82c0e0

Status changed to merged

@yorickpeterse: Wasn't clear if this actually closes #19036 (closed), but feel free to close if it does.

mentioned in issue #19355 (closed)

Removed ~149423 label

Picked into 8-9-stable, will be in 8.9.4.

mentioned in issue #19404 (closed)

mentioned in issue #19416 (closed)

mentioned in commit e2692686