remove any old file upload data on sign in

if a user signs in again, clear their file upload data from any
aborted journeys from before, so that their cookies don't fill up

also add some temporary logging when the session starts getting full.

app/main/views/send.py

@@ -1,4 +1,5 @@
 import itertools
+import json
 from string import ascii_uppercase
 from zipfile import BadZipFile
 
@@ -92,6 +93,14 @@ def get_example_letter_address(key):
 @login_required
 @user_has_permissions('send_messages', restrict_admin_usage=True)
 def send_messages(service_id, template_id):
+    # if there's lots of data in the session, lets log it for debugging purposes
+    # TODO: Remove this once we're confident we have session size under control
+    if len(session.get('file_uploads', {}).keys()) > 2:
+        current_app.logger.info('session contains large file_uploads - json_len {}, keys: {}'.format(
+            len(json.dumps(session['file_uploads'])),
+            session['file_uploads'].keys())
+        )
+
     session['sender_id'] = None
     db_template = service_api_client.get_service_template(service_id, template_id)['data']