[Part 1] Add Epic sidebar shell to refactored Epics app

What does this MR do?

This MR adds empty Epic sidebar shell in refactored Epics app which is being developed as a part of &410 (closed) (and is currently being Cookie flag that can be turned on by running document.cookie = "load_new_epic_app=true" in browser console).

Please note that this MR just lays a foundation to add more items to Epics sidebar (in multiple small MRs) and it currently just has expand/collapse button.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

#7751 (closed)