Correctly count wiki pages in sidebar

What does this MR do?

The existing code had two problems:

• We were only counting toplevel entries, not individual pages.
• We were showing the "View All Pages" button when the number of pages exactly matches the limit, even when there are no further pages.

This is now resolved by loading one page more than the limit, and using the result count to determine if we have more pages.

Ref: #212835 (closed)

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team