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
(cherry picked from commit 789d187ce5de66d72088476f8a55760e6aad47e9)
Merge "Add JSON->PHP conversion script for i18n" into wmf/1.23wmf7