Add a rake task that drops all tables

lib/tasks/gitlab/db/drop_all_tables.rake

+namespace :gitlab do
+  namespace :db do
+    task drop_all_tables: :environment do
+      connection = ActiveRecord::Base.connection
+      connection.tables.each do |table|
+        connection.drop_table(table)
+      end
+    end
+  end
+end