Fix bug where Wiki pages that include a '/' were no longer accessible

### What does this MR do?

This MR fixes a regression that caused Wiki pages that included a '/' to no longer be accessible.

### Are there points in the code the reviewer needs to double check?

Are there cases that `wiki_helper.rb` doesn't handle?

### Why was this MR needed?

The upgrade from Rails v4.1.2 to v4.1.9 (76aad9b7) caused slashes in a model ID to be escaped automatically. We can no longer use the built-in the URL helpers to generate the links for Wiki pages if we want to maintain support for slashes. There is no option to tell the formatter otherwise:

http://stackoverflow.com/questions/25031791/rails-4-1-2-to-param-escapes-slashes-and-breaks-app

The Rails code in question is here:

https://github.com/rails/rails/blob/4-1-stable/actionpack/lib/action_dispatch/journey/visitors.rb#L159

### What are the relevant issue numbers / [Feature requests](http://feedback.gitlab.com/)?

#1363

See merge request !502