It would probably be cleaner to check if we need to add the URL here rather than passing `msg` into `process_fullurl()` and checking there.

I'm not entirely happy with deciding based on the endpoint in the inbound message. I was going to suggest checking for `session['sms_offset'] == 0`, but there's a possible edge case where the URL is so long we can't send any message content and that would leave us with a loop. My next thought was to clear `session['fullurl']` after the first message, but we might need it for something else later. Do you have any other ideas? If not, I'm inclined to stay with the endpoint check and just add an explanatory comment.