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
![zngAcytBWq](/uploads/bd8514506aa5010aef9b081455ee90af/zngAcytBWq.gif)

See merge request !4448