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.
The previously failing Spinach steps have been fixed with this commit.
I have also added new steps that cover the entire usage of the Wiki
system.
The new Spinach steps revealed a minor bug in the Delete page process.
The path for the "Delete this page" button was previously set to
`project_wikis_page(@project, @wiki)` when it should have been
using the singular `project_wiki_page(@project, @wiki)` path helper.
The link has been corrected and all steps are now passing.
