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[0] 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.

[0] https://developers.google.com/analytics/devguides/collection/analyticsjs/advanced#multipletrackers
Track autoLink across both UA trackers

According to this SO answer[0], we need to use the namespace prefixing
on the linker calls as well.

[0] http://stackoverflow.com/questions/26020650/universal-analytics-cross-domain-tracking-with-multiple-trackers
Merge pull request #701 from alphagov/universal-analytics-fix

Fix double-inclusion of Universal Analytics