Docker security release! And safe deletion for orders

The docker builds were still using Django 1.11, which is old and insecure. That has been fixed in this release. Please update ASAP if using docker

Also, you now can't easily delete an Order that hasn't been completed yet.

Colour release

In this release we add a colour property to Products!
This will colour the button shown in the Order view and even sort the products by colour.

This should make it easier to quickly find the product you need.

Clear Order confirmation

This minor release just adds a confirmation dialog to the Clear Order function, to avoid accidental clearings.

None of the buttons in this dialog respond to keypresses, on purpose. ESC can be used to quickly close the dialog.

Negative Stock

In this release we add support for negative stock (via a setting) and minimum stock levels.
In the admin you can filter products to only show these with too little or enough stock.
When trying to add a product to an order while there is not enough stock, the error message is now properly displayed.

Thanks to @ZeroCoolHacker for the minimum stock functionality.

Named volumes and docker-compose 3

The database and static files are now stored in named volumes thanks to
docker-compose 3. This should make them easier to find and backup.

Nicer Admin Update (also Docker)

Thanks to @ZeroCoolHacker the admin page is now quite a bit nicer, we have added Docker support (to be improved) and we now run on newer versions of Django.

Stock bugfixes

In this release we fix some issues found by @manuelfedele
These mainly affect working with stocks on orders which are outright canceled, rather than having items removed one by one.

Tests were added for these cases.

Production-ready update!

I'm now confident in this version, with it's current feature set, it is production ready!

The "we-fixed-what-we-thought-worked" update

A couple things we thought were there. They kinda were.
There was stock tracking, but nothing stopping you from going negative and a pain to view.

We updated our dependencies, so the admin is now more mobile-friendly!

You can now print receipts! Hurray! I know, this should have been there since day one.

BREAKING major internal refactor

This is a breaking release

This means that your database WILL have to be deleted.
This is sad, but so is the nature of immature software.

Why?

Our internal system is now much more organised. It is now also easier to look at archived orders, since it's items are no longer in a long string of text.

When you delete something from an order, the item you clicked on will be deleted, not just the first one with that name.

We are getting there friends