Update convertToText.php to use new API.

Lightly tested. One issue that I anticipated that didn't eventuate
was the ordering of topics, which seems to work with the natural
ordering without any specific API parameter. Something tells me
this is actually unintended behaviour from the API.

Change-Id: I377cfdac5be7087969b300415b5b83332703f8c8
Merge "Update convertToText.php to use new API." into frontend-rewrite