Fix bug 858958 - Show dates in user specified timezone

Summary -

1. Set `USE_TZ = True`
2. Replaced `datetime.now()` with `timezone.now()` in wiki.models to make dates timezone-aware
3. Updated `datetimeformat()` template function, to display date in user timezone
4. Update profile template to show localised time

@groovecoder - Is this the right way? Still need to update tests for this.
Need to [separate top level function definitions with 2 blanks lines](http://legacy.python.org/dev/peps/pep-0008/#blank-lines).