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.

app/main/forms.py

@@ -569,19 +569,16 @@ class Triage(StripWhitespaceForm):
 
 
 class RequestToGoLiveForm(StripWhitespaceForm):
-    channel_email = BooleanField('Emails')
-    channel_sms = BooleanField('Text messages')
-    channel_letter = BooleanField('Letters')
-    start_date = StringField(
-        'When will you be ready to start sending messages?',
+    volume_email = StringField(
+        'How many emails do you expect to send in the next year?',
         validators=[DataRequired(message='Can’t be empty')]
     )
-    start_volume = StringField(
-        'How many messages do you expect to send to start with?',
+    volume_sms = StringField(
+        'How many text messages do you expect to send in the next year?',
         validators=[DataRequired(message='Can’t be empty')]
     )
-    peak_volume = StringField(
-        'Will the number of messages increase and when will that start?',
+    volume_letter = StringField(
+        'How many letters do you expect to send in the next year?',
         validators=[DataRequired(message='Can’t be empty')]
     )