Update bulk UI translations

Addresses these [3 related tickets](http://manage.dimagi.com/default.asp?144868).

Summary of changes:
- Added "platform" and "source" columns to bulk download as per the mapping provided by Sheel [here](http://manage.dimagi.com/default.asp?112688).
- Removed "default" column and it's out-of-date, hard coded, default values.
- Populate "en" column (or default language if en dne) with the default translations generated by [`app_strings.CHOICES[app.translation_strategy].get_default_translations('en')`](https://github.com/dimagi/commcare-hq/blob/bulk-ui-translations/corehq/apps/app_manager/app_strings.py#L73)