Add JSON->PHP conversion script for i18n

Apparently the .i18n.php shim is causing problems in both production
and beta labs, so for now we'll just generate the .i18n.php file
from the JSON files.

The first half of the conversion script is identical to the shim.

Change-Id: Id606584197d7efcff6fff29ea22cd35dd744b55e
Merge "Add JSON->PHP conversion script for i18n"