Externalize left sidebar strings

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")

locale/gitlab.pot

@@ -1890,6 +1890,9 @@ msgstr ""
 msgid "Close milestone"
 msgstr ""
 
+msgid "Close sidebar"
+msgstr ""
+
 msgid "Closed"
 msgstr ""