This commit combines a `tx pull` with updating the translations.json
files to change the values of those items whose key is equal to the
value. The new value is an empty string.
This comes from, in the dev environment:
./manage.py makemessages -l en -l ja
tx push -s -l ja
tx pull -f -l ja --mode=developer
The makemessages and `tx push` provide Transifex with the updated set
of strings and metadata. I'm not entirely sure why that's necessary,
but without it a lone `tx pull` left the server crashing with numerous
zerver.models.DoesNotExist exceptions.
(So to be precise, there was another `tx pull` at the start, just like
the final one. That shouldn't matter for the result, though.)