More clearly communicate that a milestone is expired

Problem to solve

As a follow up to gitlab-com/gl-infra/delivery#610 (comment 267007310), there is currently no visual indicator when selecting a milestone whether it is expired or not. This could inadvertently lead to issues and merge requests being assigned to the incorrect or outdated milestone.

Proposal

1. Move expired milestones to the bottom of the milestone list to discourage adding issues to expired milestones
2. Add (expired) next to expired milestones within the milestone select field: