Add new Epic page

What does this MR do?

Related to #10966 (closed)

The current flow to create an Epic is setting its title (and confidentiality status), and editing afterward to add the rest of the details.

This MR adds a new creation page with all the supported options (description, labels, start/due date...).

• Form data flow
• Style/Create label selector component

This page lives at /groups/[group_path]/-/epics/new, we need to input this in the browser address bar, as at the moment it doesn't have any UI access.

Screenshots

New Epic page

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