Make refactored Epic app as default

What does this MR do?

This is a final MR for &410 (closed) where we're doing following changes;

• Load new Vuex Epic app instead of older Epic app in all places where old app was used.
• Remove code of older Epic app and all its references.

Additionally this MR also fixes a small bug that's affecting new Epic app;

• Fix missing toggle sidebar button in Epic page header to show only on mobile screens.
• Fix a bug where keyboard shortcut l won't open Labels dropdown within Epic if sidebar is collapsed.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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

Closes #7751 (closed), #7753 (closed)