Add a special GitLab markdown reference for milestones

It would be nice if it was possible to reference milestones by name through a special GitLab-flavored markdown reference.

For example:

• %8.6 would link to a milestone named "8.6" in the current project
• gitlab-org/gitlab-ce%8.6 would be a cross-project reference to a milestone named "8.6" in the "gitlab-ce" project in the "gitlab-org" namespace
• %*8.6 would link to the milestone named "8.6" in the current group
• gitlab-org%8.6 would be a cross-group reference to the milestone named "8.6" in the "gitlab-org" group

(This idea was also suggested on the deprecated feedback forum.)