Remove wiki page slug dialog step when creating wiki page

What does this MR do?

Previously we asked a user to enter a new slug before taking them to the Create Page page.

As a UX improvement, we now take them to a randomly generated URI so they can begin creating their new page.

• https://gitlab.com/gitlab-org/gitlab-ce/issues/46299

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance 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