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?
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.