Add epic sidebar

What does this MR do?

Adds the sidebar for epics. Sidebar allows you to change start and end date of the epic

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

Shouldn't be

Why was this MR needed?

Deliverable

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend

TODO

• Fix styling to match design
• Clicking collapsed calendar icon should open picker (@jschatz1 mentioned we can create follow up issue for this)
• Code cleanup
• Add tests
• Backport shared components to CE

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/3556