Allow adding external URLs to the "Related issues" section.

Use cases examples are links to issues of 3rd party, external tools/libraries in github and other places where either changes need to be pushed upstream or some action is required entirely on their side.

Actual example use case: #1710850: Deploy RestWS for D7 project issue JSON that requires a link to https://github.com/dreditor/dreditor/issues/55

Another example of external related issues: #2142751-10: apostrophes in .htaccess comments should be avoided (they mess up cPanel Code Editor syntax highlighting)