Format phone numbers with spaces in download of received text messages

Some users have reported a problem with the received text message
report:

> I have tested the reply service but in the excel report the mobile
> number is showing as 4.47900E+23. How can I change the format so that
> it is show the mobile number that has replied?

This is happening because Excel is interpreting a phone number in the
format `447900900123` as a number in
[scientific notation](https://en.wikipedia.org/wiki/Scientific_notation),
in other words 4.479 &times; 10<sup>23</sup>.

`447900900123` is the format that our provider is giving us the number
in – there’s no guarantee it will always be in this format.

We can prevent this behaviour by putting spaces in the numbers. Excel
and Google Sheets won’t try to convert a string with spaces into a
number.

I think we used to do this for the sent text messages report but
probably stopped because we decided it was better to keep the phone
number in the same format as it had been supplied to us for
reconcilliation purposes.

app/main/views/dashboard.py

@@ -15,6 +15,7 @@ from flask import (
     url_for,
 )
 from flask_login import current_user
+from notifications_utils.recipients import format_phone_number_human_readable
 from werkzeug.utils import redirect
 
 from app import (
@@ -202,7 +203,7 @@ def inbox_download(service_id):
                 'Message',
                 'Received',
             ]] + [[
-                message['user_number'],
+                format_phone_number_human_readable(message['user_number']),
                 message['content'].lstrip(('=+-@')),
                 format_datetime_numeric(message['created_at']),
             ] for message in service_api_client.get_inbound_sms(service_id)['data']]