Validate date format on new platform admin page

Check that the format of the dates entered is in the `YYYY-MM-DD` as
specified and show an error message if it is not. If the date was not in
the specified format, there would be no error message but the content of
the page would change which was misleading.

app/main/views/platform_admin.py

@@ -50,9 +50,11 @@ def platform_admin():
 @login_required
 @user_is_platform_admin
 def platform_admin_new():
-    form = DateFilterForm(request.args)
+    form = DateFilterForm(request.args, meta={'csrf': False})
     api_args = {}
 
+    form.validate()
+
     if form.start_date.data:
         api_args['start_date'] = form.start_date.data
         api_args['end_date'] = form.end_date.data or datetime.utcnow().date()