Always return the translated level name.

There are many places that expect the `.level_name` method to return the
translated level name.

lib/gitlab/visibility_level.rb

@@ -89,12 +89,12 @@ module Gitlab
       end
 
       def level_name(level)
-        level_name = 'Unknown'
+        level_name = N_('VisibilityLevel|Unknown')
         options.each do |name, lvl|
           level_name = name if lvl == level.to_i
         end
 
-        level_name
+        s_(level_name)
       end
 
       def level_value(level)

locale/es/gitlab.po

@@ -7,7 +7,7 @@ msgid ""
 msgstr ""
 "Project-Id-Version: gitlab 1.0.0\n"
 "Report-Msgid-Bugs-To: \n"
-"PO-Revision-Date: 2017-07-12 12:35-0500\n"
+"PO-Revision-Date: 2017-07-13 12:10-0500\n"
 "Language-Team: Spanish\n"
 "Language: es\n"
 "MIME-Version: 1.0\n"
@@ -1059,6 +1059,9 @@ msgstr "Privado"
 msgid "VisibilityLevel|Public"
 msgstr "Público"
 
+msgid "VisibilityLevel|Unknown"
+msgstr "Desconocido"
+
 msgid "Want to see the data? Please ask an administrator for access."
 msgstr "¿Quieres ver los datos? Por favor pide acceso al administrador."
 

locale/gitlab.pot

@@ -8,8 +8,8 @@ msgid ""
 msgstr ""
 "Project-Id-Version: gitlab 1.0.0\n"
 "Report-Msgid-Bugs-To: \n"
-"POT-Creation-Date: 2017-07-12 12:31-0500\n"
-"PO-Revision-Date: 2017-07-12 12:31-0500\n"
+"POT-Creation-Date: 2017-07-13 12:07-0500\n"
+"PO-Revision-Date: 2017-07-13 12:07-0500\n"
 "Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
 "Language-Team: LANGUAGE <LL@li.org>\n"
 "Language: \n"
@@ -1060,6 +1060,9 @@ msgstr ""
 msgid "VisibilityLevel|Public"
 msgstr ""
 
+msgid "VisibilityLevel|Unknown"
+msgstr ""
+
 msgid "Want to see the data? Please ask an administrator for access."
 msgstr ""