Disable i18n machinery, use correct locale

Disable i18n machinery, use correct locale

Two issues here. Firstly, the use of the 'USE_I18N'. The Django docs
describe this as such:

  A boolean that specifies whether Django’s translation system should
  be enabled. This provides an easy way to turn it off, for performance.
  If this is set to False, Django will make some optimizations so as not
  to load the translation machinery.

We don't do translations and won't until such a time as someone comes
asking for them. Optimize things accordingly by setting 'USE_I18N' to
False and removing the now-unnecessary 'LANGUAGE_CODE' setting.

Secondly, the use of en-AU is a bit of a lie since our UI is actually
written in US English (or should be). The primary reason for a lang tag
to be present is to assist screenreaders and other accessibility tools,
so make their lives easier by reflecting the truth.

Signed-off-by: Stephen Finucane <stephen@that.guru>