Add milestone expire date to the right sidebar

What does this MR do?

Shows milestone expire date tooltip in the sidebar. It also shows the tooltip when sidebar is collapsed.

Are there points in the code the reviewer needs to double check?

common_utils.js.coffee and milestones_helper.rb should be reviewed deeply.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/17355

Screencast