i18n: Adjust Transifex sync-translations download mode.
i18n: Adjust Transifex sync-translations download mode.

It appears that some server-side change to Transifex resulted in the
"onlytranslated" mode deleting some (all?) strings from django.po files that
were not translated.

Testing determined that the "translator" mode appears to now be the
only mode that works with both our django.po and translations.json
files (We want to avoid both copying the English strings and deleting
strings), so we're switching to that.

Background is available here:
https://chat.zulip.org/#narrow/stream/3-backend/topic/4.2Ex.20branch.20translations.20sync/near/1187324

hackerkid/zulip,hackerkid/zulip,kou/zulip,hackerkid/zulip,rht/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,kou/zulip,punchagan/zulip,punchagan/zulip,zulip/zulip,rht/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,punchagan/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,punchagan/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,kou/zulip,zulip/zulip,zulip/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip