Replace channels and volumes with volumes/channel

Now that we’re a more mature platform we don’t care so much about the
load that one service might put on our platform.

We do care about intended volumes for two reasons:
- modelling the benefits that services get from using Notify
- managing stocks of envelopes (while our letter volumes are small
  enough that they could be skewed by one new service)

Changing to the ‘how many per year’ question also has the benefit of
mapping directly to the data we store in the ‘beta partners’
spreadsheet.

tests/app/main/views/test_service_settings.py

@@ -691,12 +691,21 @@ def test_should_show_request_to_go_live(
     )
     assert page.h1.text == 'Request to go live'
     for channel, label in (
-        ('email', 'Emails'),
-        ('sms', 'Text messages'),
-        ('letter', 'Letters'),
+        (
+            'email',
+            'How many emails do you expect to send in the next year? For example, 1,000,000'
+        ),
+        (
+            'sms',
+            'How many text messages do you expect to send in the next year? For example, 500,000'
+        ),
+        (
+            'letter',
+            'How many letters do you expect to send in the next year? For example, 5,000'
+        ),
     ):
         assert normalize_spaces(
-            page.select_one('label[for=channel_{}]'.format(channel)).text
+            page.select_one('label[for=volume_{}]'.format(channel)).text
         ) == label
 
 
@@ -716,11 +725,9 @@ def test_should_redirect_after_request_to_go_live(
         'main.submit_request_to_go_live',
         service_id=SERVICE_ONE_ID,
         _data={
-            'channel_email': 'y',
-            'channel_sms': 'y',
-            'start_date': '01/01/2017',
-            'start_volume': '100,000',
-            'peak_volume': '2,000,000',
+            'volume_email': '111',
+            'volume_sms': '222',
+            'volume_letter': '333',
         },
         _follow_redirects=True
     )
@@ -738,13 +745,12 @@ def test_should_redirect_after_request_to_go_live(
         '---\n'
         'Organisation type: central\n'
         'Agreement signed: Can’t tell (domain is user.gov.uk)\n'
-        'Channel: email and text messages\n'
-        'Start date: 01/01/2017\n'
-        'Start volume: 100,000\n'
-        'Peak volume: 2,000,000\n'
+        'Emails in next year: 111\n'
+        'Text messages in next year: 222\n'
+        'Letters in next year: 333\n'
         '\n'
         '---\n'
-        '{}\tNone\tservice one\tTest User\ttest@user.gov.uk\t-\t21/12/2012'
+        '{}\tNone\tservice one\tTest User\ttest@user.gov.uk\t-\t21/12/2012\t222\t111\t333'
     ).format(SERVICE_ONE_ID, SERVICE_ONE_ID)
 
     assert normalize_spaces(page.select_one('.banner-default').text) == (