Add "New epic" button within epic page

What does this MR do?

Related to #211533 (closed)

As we're moving to use a new epic creation page, this MR adds a button that links to that page on epic pages, similar to the "New issue" button on issues.

The UI related to this MR is under a feature flag: :create_epic_form.

Screenshots

Before	After	After (mobile)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done