Update content_class for snippets form pages

**How?**
This just adds the `@content_class` setup which
we have peppered throughout our views...

app/views/projects/snippets/edit.html.haml

 - add_to_breadcrumbs _("Snippets"), project_snippets_path(@project)
 - breadcrumb_title @snippet.to_reference
 - page_title _("Edit"), "#{@snippet.title} (#{@snippet.to_reference})", _("Snippets")
+- @content_class = "limit-container-width" unless fluid_layout
 
 %h3.page-title
   = _("Edit Snippet")

app/views/projects/snippets/new.html.haml

 - add_to_breadcrumbs _("Snippets"), project_snippets_path(@project)
 - breadcrumb_title _("New")
 - page_title _("New Snippet")
+- @content_class = "limit-container-width" unless fluid_layout
 
 %h3.page-title
   = _("New Snippet")

app/views/snippets/edit.html.haml

 - page_title _("Edit"), "#{@snippet.title} (#{@snippet.to_reference})", _("Snippets")
+- @content_class = "limit-container-width" unless fluid_layout
 
 %h3.page-title
   = _("Edit Snippet")

app/views/snippets/new.html.haml

 - @hide_top_links = true
 - @hide_breadcrumbs = true
 - page_title _("New Snippet")
+- @content_class = "limit-container-width" unless fluid_layout
 
 .page-title-holder.d-flex.align-items-center
   %h1.page-title= _('New Snippet')