Check for availability of ga in script

[st3v3nhunt]

In customga.js it is assumed ga is available as a global function. This is only the case when Google Analytics has been loaded. Currently there is a check in the view that includes the JS. Ideally the JS would do the check itself to prevent it being run in an environment where ga should be available but for whatever reason isn't.

This would also allow all of the customXX.js files to be combined into a single one which could then be minified and hashed.