Issue Parent-Target indicator via CSS or similar

Linking between issues and target issue / project works as this documentation says: https://docs.gitlab.com/ee/api/issue_links.html#create-an-issue-link

Meanwhile #2036 (closed) tries to have child-parent relations. I ask for nothing but a CSS-class added soon, becaue it makes things much more easier..

If the related issue is the source, then the entry or link has an additional css class (e.g. "target"). If the current issue is the source and the other is the target, then another css class (e.g. "source").

By doing this simple step everyone is able to distinguish using JS or custom CSS and add proper signs/icons to each linked issue and identify the right way.