Fixing reverse for form_submitted()

[handelaar]

Using django.http.HTTP.HttpResponseRedirect() and django.urls.reverse()

Would fix #285 (does for me anyway)

[handelaar]

Fixed the travis error for Flake8. Remaining problems with checks are.... a bit old.

[dracos]

All django.shortcuts.redirect does is call reverse, as you can see at https://github.com/django/django/blob/master/django/shortcuts.py#L150
So I would assume that doing nothing but changing 'mapit.views.postcodes.postcode' to 'mapit-postcode' would also fix the issue, and not have any older-Django-version issues :) I have now found in the Django 1.10 release notes: "The ability to reverse() URLs using a dotted Python path is removed." so that's the issue.

It would be good to have a test added showing that this fails in 1.10/11 – Travis automatically runs the tests on 1.8-1.11. Django 1.8 is supported until early 2018, and so we support it also.