fix one more place where senders weren't sanitised

make sure everything is using the `nl2br` formatter that properly wraps
it in markdown to keep everything sanitised nicely. Also write a couple
of tests

app/main/views/send.py

@@ -31,6 +31,7 @@ from xlrd.xldate import XLDateError
 from app import (
     current_service,
     job_api_client,
+    nl2br,
     notification_api_client,
     service_api_client,
 )
@@ -272,7 +273,7 @@ def get_sender_context(sender_details, template_type):
         if context['default_id'] == context.get('receives_text_message', None):
             context['default_and_receives'] = context['default_id']
 
-    context['value_and_label'] = [(sender['id'], sender[sender_format]) for sender in sender_details]
+    context['value_and_label'] = [(sender['id'], nl2br(sender[sender_format])) for sender in sender_details]
     return context