Refactored Epic app shell

What does this MR do?

This MR adds refactored Epic app shell (excl. Sidebar) written with Vuex. Points to keep in mind with this MR;

• App is currently behind a cookie flag and can be enabled by running document.cookie = "load_new_epic_app=true" in browser console which will replace current Epics app with new app.
• Visually the new app is not different from the old one, it is behind the scenes implementation that's rewritten from ground up.
• New app does not include sidebar as it is to be covered as a part of https://gitlab.com/gitlab-org/gitlab-ee/issues/7751.

Learn more about complete scope of this effort in &410 (closed).

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• Link to e2e tests MR added if this MR has ~"Requires e2 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?

Closes #7750 (closed)