Merge branch 'patch-54' into 'master'

Externalize left sidebar strings

See merge request gitlab-org/gitlab-ce!27532

app/views/shared/_sidebar_toggle_button.html.haml

 %a.toggle-sidebar-button.js-toggle-sidebar{ role: "button", type: "button", title: "Toggle sidebar" }
   = sprite_icon('angle-double-left', css_class: 'icon-angle-double-left')
   = sprite_icon('angle-double-right', css_class: 'icon-angle-double-right')
-  %span.collapse-text Collapse sidebar
+  %span.collapse-text _("Collapse sidebar")
 
 = button_tag class: 'close-nav-button', type: 'button' do
   = sprite_icon('close', size: 16)
-  %span.collapse-text Close sidebar
+  %span.collapse-text _("Close sidebar")