Show Group level Roadmap

What does this MR do?

Adds Group level Roadmap based on Epics for a group that have start or end dates specified which falls in current timeframe where timeframe is a 6 months window before & after today.

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

Following files are suggested to be reviewed briefly;

• ee/app/assets/javascripts/roadmap/components/epics_list_section.vue
• ee/app/assets/javascripts/roadmap/components/epic_item_timeline.vue

Why was this MR needed?

With Epics having start and end dates, it is nice to be able to visualize timeline for an Epic in graphical form, and Roadmap view enables the same.

Screenshots

Default View	Epics from Subgroups	Epics Scrollable List	Empty State with No Epics	Empty State with No Epics in Current Timeframe

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• [ ] End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #3559 (closed)