short_message: use MULTITEXT message type by default

Only mobyt support this parameter, so this break the compatibility with
essendex

bt5/erp5_short_message/SkinTemplateItem/portal_skins/erp5_short_message/ShortMessage_send.xml

@@ -91,7 +91,7 @@ context.portal_sms.activate(\n
   recipient=to_url,\n
   sender=from_url,\n
   sender_title=from_title,\n
-  message_type="text",\n
+  message_type="MULTITEXT",\n
   test=download,\n
   document_relative_url=context.getRelativeUrl(),\n
   **kw)\n

[Jérome Perrin]

Some notes:

With mobyt, messages with more than 140 characters are rejected when using text mode, if we use MULTITEXT, multiple messages are sent. This also means that the cost of sending such message is higher.

Probably we want a way to configure we want to send only one message and fail if messages are too long instead of sending multuiple messages and increase the cost of sending.