Add Google Analytics property to .env file

[tompollard]

There is a Google Analytics property hardcoded in the following template:

https://github.com/MIT-LCP/physionet-build/blob/303edbb43b594fa13ff3f6bf704c68eab6873cc8/physionet-django/templates/base_js_top.html#L1-L11

This property should be configurable. Add a GOOGLE_ANALYTICS_ID to the .env file. If the ID is not provided, the header should not include the Google Analytics chunks.