fix 500 errors with excel files > 500k size limit

werkzeug's internal workings keep files under 500kb in memory, and files
greater than 500kb as a TemporaryFile

(https://github.com/pallets/werkzeug/blob/0.11-maintenance/werkzeug/formparser.py#L38)

when we encounter a CSV or TSV, we call normalise_newlines, which invokes
`.read()`, however when we were passing straight into pyexcel, we called
`file.getvalue()` - this exists on BytesIO (small files) but not on
TemporaryFile objects (large files) - we were seeing 500 errors

tests/app/test_utils.py

@@ -1,9 +1,12 @@
-import pytest
+from pathlib import Path
 from io import StringIO
-from app.utils import email_safe, generate_notifications_csv, generate_previous_dict, generate_next_dict
 from csv import DictReader
+
+import pytest
 from freezegun import freeze_time
 
+from app.utils import email_safe, generate_notifications_csv, generate_previous_dict, generate_next_dict, Spreadsheet
+
 
 def test_email_safe_return_dot_separated_email_domain():
     test_name = 'SOME service  with+stuff+ b123'
@@ -67,3 +70,9 @@ def test_generate_next_dict(client):
 def test_generate_previous_next_dict_adds_other_url_args(client):
     ret = generate_next_dict('main.view_notifications', 'foo', 2, {'message_type': 'blah'})
     assert 'notifications/blah' in ret['url']
+
+
+def test_can_create_spreadsheet_from_large_excel_file():
+    with open(str(Path.cwd() / 'tests' / 'spreadsheet_files' / 'excel 2007.xlsx'), 'rb') as xl:
+        ret = Spreadsheet.from_file(xl, filename='xl.xlsx')
+    assert ret.as_csv_data