Fix double-inclusion of Universal Analytics

Since https://github.com/alphagov/static/pull/508 was merged the Universal Analytics code used to support cross-transaction tracking would be broken.

This commit updates the in-template code as follows:

- Remove dynamic creation of Javascript loader
- Use [UA namespacing](https://developers.google.com/analytics/devguides/collection/analyticsjs/advanced#multipletrackers) to namespace the `pageview` beacon sending functionality

This commit will need to be released at the same time as the code in the above pull request to avoid any production issues.

CC @edds @dsingleton