Add ContactSubmission model and use it to ContactMixin submissions.

[solarissmoke]

There is one decision here which is possibly contentious - whether to do this in the view or the form.

[craigloftus]

I don't have a particular problem with this approach, but I think I would have done it in the form save method. What was your rational for doing otherwise?

You do need to specify the postgres version .travis.yml to get the build to pass.

addons:
  postgresql: "9.6"

[solarissmoke]

See comment here: https://github.com/regulusweb/wagtail-extensions/pull/30/files#diff-3cbffe09025c1312b8bb0d98ce858d02R37

The idea was that even if a project is using it's own form class it can still have the data saved without having to modify the corresponding form logic.