Update to Django 2.2

Upgrade site and modules to Django 2.2. Remove and replace obsolete
functionality with current equivalents. Update requirements to latest
versions where possible. Remove unused dependencies.

Add button to update invoice

Allow the recipient on an invoice to be updated without creating a new invoice.
This allows fixing of mistakes easily, especially for when it is given to someone
else to pay and a new invoice would be confusing.

Finalise registration

Add shirt types and sizes. Improve messaging about discounts.
Restyle ticket wizard and product category screens. Enable page titles
and messages. Update dashboard to hide raffle.
Enable inventory population for dev container.

The 2018ificiation

I think this removes most references to "hobart", "pycon", and "2017"

There are still some references to some images that we don't have a
replacement for.

Simplify django form rendering

Remove my hack to get bootstrap forms.

Remove the insane render-row-at-a-time forms that then only render
default django forms.

Adds registration to the website (#69)

* Updates settings and requirements

* First pass at attendee profile

* Imports the registration templates; defines attendee profile models etc.

* First pass at themeing the registration form.

* First page of the registration form: done!

* Makes form validation nicer

* Adds populate_inventory

* Improves the additional items page

* Allows for rendering of formsets.

* Adds support for formset extending.

* Removes formset delete buttons

* Review page is LCA-ified

* Fixes some formset behaviour

* Fixes urls.py

* LCA-ifies product_category.html

* Invoices

* Credit card payments

* s/register/tickets/

* Show registration features only whilst products are available (think about this better, later)

* Updates the attendee profile form page

* Form tidy-up

* Makes it so that address info is copied from attendee profile to the address details are autofilled in Stripe.

* Adds feature to offer Australians a dropdown list of states rather than free text.

* Allow toggling of void invoices.

* Adds backgrounds to the headers in the registration process

* Improves the review page

* Adds “Linux Australia” to invoice details.

* Do not show balance due on void/refunded invoices.

* More thumbing

* Adds a link back to reports on each report.

* Tokenisation language.

* Another bug in credit card processing.

* Adds stripe refunds to options

* Removes spurious dashboard button.

* Tidies up the presentation of discounts.

* Tidies up presentation of voucher form.

* Fixes sponsor logo appearance with adblock.

* Front page tweaks

* Lets us specify alternative URLs in homepage panels

* more

* Updates discount amounts.

* More website fixes

* Changes language on pay invoice button

* Adds contact details to the invoice template.

* Updates the currency message in the invoice template.

* Explicitly includes e-mail address, because theme_contact_email doesn’t propagate

* Changes payment text.

* s/registration/selections/

* Removes final face palm

* Fixes lack of speaker dinner tickets for actual presenters.

* Adjusts wording in invoice e-mails

* Invoice wording.

* (FIX)

* Fixes margins on lists and tables

* Improvements arising from those CSS fixes.

* Changes description tags.