Give the user better error messages for CSV files

Makes uses of the additions to utils in https://github.com/alphagov/notifications-utils/pull/9

This commit strips out a lot of the complex stuff that the views and templates
in this app were doing. There is now a cleaner separation of concerns:

- utils returns the number and type of errors in the csv
- `get_errors_for_csv` helper in this app maps the number and type of errors
  onto human-friendly error messages
- the view and template just doing the glueing-together of all the pieces

This is (hopefully) easier to understand, definitely makes the component
parts easier to test in isolation, and makes it easier to give more specific
error messages.

tests/app/main/test_errors_for_csv.py

@@ -0,0 +1,79 @@
+from collections import namedtuple
+
+import pytest
+
+from app.utils import get_errors_for_csv
+
+
+MockRecipients = namedtuple(
+    'RecipientCSV',
+    [
+        'missing_column_headers',
+        'rows_with_bad_recipients',
+        'rows_with_missing_data'
+    ]
+)
+
+
+@pytest.mark.parametrize(
+    "missing_column_headers,rows_with_bad_recipients,rows_with_missing_data,template_type,expected_errors",
+    [
+        (
+            [], [], [],
+            'sms',
+            []
+        ),
+        (
+            [], {2}, [],
+            'sms',
+            ['fix 1 phone number']
+        ),
+        (
+            [], {2, 4, 6}, [],
+            'sms',
+            ['fix 3 phone numbers']
+        ),
+        (
+            [], {1}, [],
+            'email',
+            ['fix 1 email address']
+        ),
+        (
+            [], {2, 4, 6}, [],
+            'email',
+            ['fix 3 email addresses']
+        ),
+        (
+            ['name'], {2}, {3},
+            'sms',
+            [
+                'add a column called ‘name’',
+                'fix 1 phone number',
+                'fill in 1 empty cell'
+            ]
+        ),
+        (
+            ['name', 'date'], [], [],
+            'sms',
+            ['add 2 columns, ‘name’ and ‘date’']
+        ),
+        (
+            ['name', 'date', 'time'], {2, 4, 6, 8}, {3, 6, 9, 12},
+            'sms',
+            [
+                'add columns called ‘name’, ‘date’, and ‘time’',
+                'fix 4 phone numbers',
+                'fill in 4 empty cells'
+            ]
+        )
+    ]
+)
+def test_get_errors_for_csv(
+    missing_column_headers, rows_with_bad_recipients, rows_with_missing_data,
+    template_type,
+    expected_errors
+):
+    assert get_errors_for_csv(
+        MockRecipients(missing_column_headers, rows_with_bad_recipients, rows_with_missing_data),
+        template_type
+    ) == expected_errors