Fix the Cancel button on the Edit Wiki page.

The Cancel button on the Edit Wiki page was still redirecting back
to the "Index" page which is no longer the default Wiki page.

This commit changes the Cancel button in the following ways:

  * Pressing Cancel while editing an existing Wiki page will now
    redirect you back to the latest version of that page.
  * Pressing Cancel while editing a brand new Wiki home page that
    does not yet exist will redirect you to back to the same Edit
    Wiki Home page.