add ldap group links migrations

db/migrate/20170911133813_drop_cn_constraint_to_ldap_group_links.rb

+class DropCnConstraintToLdapGroupLinks < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def change
+    change_column_null :ldap_group_links, :cn, true
+  end
+end

db/migrate/20170911134018_add_filter_to_ldap_group_links.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddFilterToLdapGroupLinks < ActiveRecord::Migration
+  def change
+    add_column(:ldap_group_links, :filter, :string)
+  end
+end

db/schema.rb

@@ -941,12 +941,13 @@ ActiveRecord::Schema.define(version: 20170918223303) do
   add_index "labels", ["type", "project_id"], name: "index_labels_on_type_and_project_id", using: :btree
 
   create_table "ldap_group_links", force: :cascade do |t|
-    t.string "cn", null: false
+    t.string "cn"
     t.integer "group_access", null: false
     t.integer "group_id", null: false
     t.datetime "created_at"
     t.datetime "updated_at"
     t.string "provider"
+    t.string "filter"
   end
 
   create_table "lfs_objects", force: :cascade do |t|