Define and visualize useful/waste time

Overview

Value Stream Management diagrams sometimes depict work flowing through a value stream in a binary state: it's either being worked on actively by a responsible stage (e.g. actively developed), or it's sitting idle in some way waiting for the next stage (e.g. waiting on a code reviewer to become available).

We should depict this in GitLab.

Proposal

• Allow a group to define issue labels into "Idle" and "Useful" segments.
• Measure the amount of time that issues spend in each segment, respectively.
• Provide a stacked bar visualization (possibly a waterfall bar chart) over time.
• Highlight which areas have increased/decreased over time.

cc @gweaver