Confluence nav item links to landing page

What does this MR do?

This merge request changes the Confluence nav item to link to a new landing page. From the landing page, the user can continue to their Confluence Workspace.

• Issue: #220934 (closed)

QA steps:

1. Enable the :confluence_integration feature flag
2. Visit a project
3. Go Settings > Integrations
4. Choose the Confluence integration
5. Paste in the URL http://gitlabknowledge.atlassian.net/wiki/home
6. Save the integration to enable it
7. A Confluence nav item will appear on the side bar (replaces the Wiki one)
8. Click it
9. You should see the landing page
10. Click the Go to Confluence button to visit the Confluence Workspace

Screenshots

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