Don't use `params[:id]` while building `markdown_preview_path`.

1. Instead, use `@page.title`, since it's always guaranteed to be around
   in a wiki.

2. `params[:id]` is _not_ always guaranteed to be around - if a page is
    created with errors, `render :edit` is called, and `params[:id]` is
    `nil`.

3. More context: https://gitlab.com/gitlab-org/gitlab-ce/issues/20079#note_13223240

app/views/layouts/project.html.haml

@@ -6,7 +6,7 @@
 - content_for :scripts_body_top do
   - project = @target_project || @project
   - if @project_wiki && @page
-    - markdown_preview_path = namespace_project_wiki_markdown_preview_path(project.namespace, project, params[:id])
+    - markdown_preview_path = namespace_project_wiki_markdown_preview_path(project.namespace, project, @page.title)
   - else
     - markdown_preview_path = markdown_preview_namespace_project_path(project.namespace, project)
   - if current_user