Make categories static instead of calling the API to get them (bug 968312)

https://bugzilla.mozilla.org/show_bug.cgi?id=968312

This completely removes the API call to fetch categories in fireplace. Instead, they are hardcoded in a javascript file (since we don't change them much). This should improve perf since it removes one external HTTP request.

Once it's merged, the plan for translations is to:
- export and merge POT/PO files without committing yet
- use `generate_categories_translations.py` included in that PR to dump over existing translations into the .po file for each locale
- commit everything, notifying @mathjazz first - and make sure Verbatim updates correctly