Update db/structure.sql to new format

As of Rails 4.0, db:structure:dump now uses mysqldump rather than a
custom format:

    https://github.com/rails/rails/commit/ccc6910cb034efc6b749e1ae82c748085a671fa9

It now includes a lot more detail - this commit doesn't include any
actual schema changes.
Merge pull request #375 from alphagov/update_structure_format

Update db/structure.sql to new format