Merge request sidebar missing milestone tooltip when not set

Some of the icons on the MR sidebar aren't immediately intuitive, so they should all have tool tips.

If the milestone is not set, no tooltip is shown.

Proposal

Show Milestone as the the milestone tooltip if the right side bar is collapsed and no milestone is set.

Relevant logs and/or screenshots

HN comment: https://news.ycombinator.com/item?id=14358091