New epic button in Epic Roadmap empty state should direct the user to a full epic creation page

What does this MR do?

Related to #229621 (closed)

• Replace epic-create popover component with link
• Remove ee/app/assets/javascripts/epic/components/epic_create.vue
• Update tests to be able to use provide/inject

Screenshots (strongly suggested)

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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