Merge branch 're-add-kerberos' into 'master'

Re-add Kerberos to AuthHelper form-based providers



See merge request !40

app/helpers/auth_helper.rb

 module AuthHelper
   PROVIDERS_WITH_ICONS = %w(twitter github gitlab bitbucket google_oauth2).freeze
-  FORM_BASED_PROVIDERS = [/\Aldap/, 'crowd'].freeze
+  FORM_BASED_PROVIDERS = [/\Aldap/, 'kerberos', 'crowd'].freeze
 
   def ldap_enabled?
     Gitlab.config.ldap.enabled