A VisibilityLevel also can be presented as string

For the API, the VisibilityLevel will be exposed as String instead of
Integer. So add the string values and method to translate a level
integer to a string.

lib/gitlab/visibility_level.rb

@@ -35,6 +35,10 @@ module Gitlab
     class << self
       delegate :values, to: :options
 
+      def string_values
+        string_options.keys
+      end
+
       def options
         {
           'Private'  => PRIVATE,
@@ -43,6 +47,14 @@ module Gitlab
         }
       end
 
+      def string_options
+        {
+          'private'  => PRIVATE,
+          'internal' => INTERNAL,
+          'public'   => PUBLIC
+        }
+      end
+
       def highest_allowed_level
         restricted_levels = current_application_settings.restricted_visibility_levels
 
@@ -82,6 +94,10 @@ module Gitlab
 
         level_name
       end
+
+      def string_level(level)
+        string_options.key(level)
+      end
     end
 
     def private?