Add a migration to remove soft-deleted groups.

The database should not have any soft-deleted groups. Due to a race
condition (soft-delete completes after the hard-delete), soft-deleted
groups were (incorrectly) left in the database, causing issues while
trying to create a new group with the same name.

db/migrate/20161117114805_remove_undeleted_groups.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class RemoveUndeletedGroups < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def up
+    execute "DELETE FROM namespaces WHERE deleted_at IS NOT NULL;"
+  end
+
+  def down
+    # This is an irreversible migration;
+    # If someone is trying to rollback for other reasons, we should not throw an Exception.
+    # raise ActiveRecord::IrreversibleMigration
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20161113184239) do
+ActiveRecord::Schema.define(version: 20161117114805) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"