milestones be friendly url

currently roadmap urls use internal id in urls:

• https://gitlab.com/gitlab-org/gitlab-ce/milestones/24 - milestone named 8.8

but it could just be the milestone name itself there, like:

• https://github.com/eventum/eventum/milestones/3.1.2 - milestone named 3.1.2

this would improve usability and be easily understand what the link is (and even spot mistakes) when links used outside gitlab (emails, wikis, intranets)