Validate CSVs fully

This commit extends the existing function to validate each row’s phone number
to also validate that all the required data is present.

It does this using the checking that the `Template` class can do when given
a template and a `dict` of values.

app/templates/views/send-sms.html

@@ -22,13 +22,6 @@
 
     {{file_upload(form.file, button_text='Choose a CSV file')}}
 
-    {{ banner(
-      'You can only send messages to yourself until you <a href="{}">request to go live</a>'.format(
-        url_for('.service_request_to_go_live', service_id=service_id)
-      )|safe,
-      type='important'
-    ) }}
-
     {{ page_footer(
       "Continue to preview"
     ) }}
@@ -36,10 +29,10 @@
     {% if column_headers %}
       {% call(item) list_table(
         example_data,
-        caption='Preview',
+        caption='Example',
         field_headings=column_headers,
         field_headings_visible=True,
-        caption_visible=False,
+        caption_visible=True,
         empty_message="Your data here"
       ) %}
         {% call field() %}