12905b - Support design tab link references

What does this MR do?

This MR includes the changes to support link references to the design tab for issues. This one of several MRs created to close #12905 (closed), and this one addresses the need to handle the URLs of the design tab.

Prior to these changes, URLs such as https://gitlab.com/gitlab-org/gitlab/issues/13495/designs would be found by the Issue reference filter, and the inner-html would be set to #13495.

With these changes, that same URL would also be found and processed, but now would have its inner HTML set to: #13495 (designs), using the object_link_text_extras hook in AbstractReferenceFilter.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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

See #12905 (closed)