Visually distinguish epics using colors
### Release Notes
To further improve the ability to use portfolio management features across the organization, you can now [distinguish epics using colors](https://docs.gitlab.com/ee/user/group/epics/manage_epics.html#epic-color) on roadmaps and epic boards. Quickly distinguish between group ownership, stage in a lifecycle, development towards maturity, or a number of other categorizations with this lightweight but versatile feature.

## Summary
To help distinguish epics on Roadmap and Epic Boards, we want to allow assigning a color to an epic and display that color when viewing epics.
Behind `:epic_color_highlight` feature flag.
| Roadmap | Epic Board | Edit color in epic sidebar |
|---------|------------|----------------------------|
|  |  |  |
epic