Tags by type and ID

Change tag URLs from `/tags/<name>.json` to `/tags/<type>/<name>.json` (with redirects from the old URLs).

The `with_tag` handler also now identifies tags by their type and ID (see [examples in the comments](https://github.com/alphagov/govuk_content_api/blob/04fed8ac7073fc8b3a12f07b2038112b5aa0b0ef/govuk_content_api.rb#L229-238)).

We shouldn’t deploy this until alphagov/whitehall#175 **(done)** and alphagov/frontend#212 **(also done)** are both deployed, as otherwise we’ll break anything that depends on the old URLs and can’t handle redirects.