settings: Ensure GOOGLE_ANALYTICS_* available when compressing

Context processors are not available when compressing offline. This
makes sure that the offline compression and the online asset lookup
agree on the content of the compressed JavaScript, by ensuring that the
Google Analytics code is rendered the same in both cases.

See:

- https://twitter.com/jezdez/status/435768328305782784
- http://django-compressor.readthedocs.org/en/latest/settings/#django.conf.settings.COMPRESS_OFFLINE_CONTEXT