Additional LDAP Authentication Methods (#1287)

This patch allows anonymous and user based authentication to LDAP
servers. This change is based on a patch against bn-ldap-authentication:

-  https://github.com/blindsidenetworks/bn-ldap-authentication/pull/2

The patch introduces a new environment variable `LDAP_AUTH` which
controls the authentication method used against the LDAP server:

- `anonymous` enables an anonymous bind to the LDAP with no password
  being used.

- `user` uses the user's own credentials to search for his data,
  enabling authenticated login to LDAP without the need for a user with
  global read privileges.

The default still remains at using a bind user, allowing for a seamless
upgrade path from the previous version.

This fixes #1082

Co-authored-by: Jesus Federico <jesus@123it.ca>

config/initializers/omniauth.rb

@@ -11,8 +11,7 @@ Rails.application.config.providers = []
 # Set which providers are configured.
 Rails.application.config.omniauth_bn_launcher = Rails.configuration.loadbalanced_configuration
 Rails.application.config.omniauth_ldap = ENV['LDAP_SERVER'].present? && ENV['LDAP_UID'].present? &&
-                                         ENV['LDAP_BASE'].present? && ENV['LDAP_BIND_DN'].present? &&
-                                         ENV['LDAP_PASSWORD'].present?
+                                         ENV['LDAP_BASE'].present?
 Rails.application.config.omniauth_twitter = ENV['TWITTER_ID'].present? && ENV['TWITTER_SECRET'].present?
 Rails.application.config.omniauth_google = ENV['GOOGLE_OAUTH2_ID'].present? && ENV['GOOGLE_OAUTH2_SECRET'].present?
 Rails.application.config.omniauth_office365 = ENV['OFFICE365_KEY'].present? &&