Improve collapsed sidebar tooltips consistency

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Follow-up to discussion in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23379#note_121254703

When the sidebar is collapsed, the tooltips are as follows:

1. Milestones

   • When the value is empty / none: Milestone
   • When the value has no due date: 11.6<br>Milestone
   • Else: 11.6<br>2018-12-22 (xx days remaining)

2. Due date

   • When the value is empty / none: Due date
   • Else: Due Date<br>2018-12-07 (xx days remaining)

3. All other fields

   • When the value is empty / none: Field name
   • Else: Field value

Some inconsistencies:

1. Should we show the field name for all fields? Or maybe the current behavior is correct where we show the field name for some fields, maybe those that are not so easily recognized by their icon.

2. For the fields with labels (Milestone and Due Date) the order is inconsistent. Value<br>Milestone vs Due Date<br>value