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.

Improved handling of static/build and css generation

When the container is mounted, the local contents of . obscure
/app/symposion_app in the image.

Generally speaking, this is handy for development, as it means that
local changes are detected and used immediately without needing to
restart the container.

However, it breaks in the specific case of the sass->css
generation. Prior to this change, the css is generated only after the
first time a page is hit. The generated file is placed in
static/build; however, due to the obscuration, this generated file
isn't visible to the running process.

The next time the container is built, the pre-existing static/build
directory is copied into the container as it's being built; then,
later, that version is what gets served.

This change adds the needed libraries to pre-generate the css as part
of the image build, and runs compilescss to do this, prior to the
collectstatic step.

It also adds a second collectstatic into the make_dev_container
script, so that the visible static/build directory should ahve the
same contents as the obscured static/build directory. It also expands
the .dockerignore file to make sure these files aren't copied into the
image in future.

I'm not sure if this is the right thing to do, as changes to this
directory will be ignored, which could be confusing. Perhaps never
being able to see these generated files is better?

Modify dependencies to suit debian stretch

The python:3.6 tag now points to stretch.

Changed the name of the base image to make this more explicit.

Make our fonts available to the system

To make them more useable by cairo

Increase uwsgi buffer size

The DataTables sets a lot of data into some cookies. It's currently
overwhelming the uwsgi buffer size.

Utilise multi-stage docker build for consistent dev/prod base

* developers can use --target symposion_dev to get a responsive site
that reads from their homedir and reacts to changed files instantly
* without a specified target the default is to build the prod image,
which is identical except for running uwsgi instead of the django
built-in server
* Enable debug when running in a developer's test environment
* Remove the makemigrations script and dockerfile

Reorder dockerfile

This creates more intermediates, but moves those that are less likely
to change to the top of the file. In theory this will produce faster
builds on a developer's machine as they won't need to apt-get update
every time.

Collect Static so admin works

Thanks tchaypo and jammielennox

Closes #11

Add initial Dockerfile

This builds a working docker image for the application.
This is only an initial revision. It needs testing. MySQL, PIL, etc.

Was going to use Alpine, however, python3.6 segfaulted on docker
starting up, and I couldn't find the core files...