Reword choose sender/reply to address pages

Make it clear that:

- In the case of text messages, it’s about who the message comes from
- In the case of emails, it’s about where the user will reply to

tests/app/main/views/test_send.py

@@ -45,13 +45,13 @@ test_non_spreadsheet_files = glob(path.join('tests', 'non_spreadsheet_files', '*
         mock_get_service_email_template,
         multiple_reply_to_email_addresses,
         'Choose where to send replies',
-        'Select an email address that recipients can reply to'
+        'Where should replies go?',
     ),
     (
         mock_get_service_template,
         multiple_sms_senders,
         'Chose text message sender',
-        'Select a text message sender that the recipients can reply to'
+        'Who should the message come from?'
     )
 ])
 def test_show_correct_title_and_description_for_sender_type(