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

make_dev_container

Expand on support for devs by scripting the process of creating,
starting, and initialising a dev container.

Take container name as an optional parameter

Update testing fixtures to add admin users

This allows testing deployments to have accounts that can log in as
admin.

Correct some spelling in README

README had some typos, correct them.

Add a note to get the permission required for papers review

So not obvious...

Specify python3 when creating a virtualenv in README

This app is now python3 only so we must specify python3 when creating a
virtualenv. Yes this assumes that most people are using python2 as
default, most distros still ship this way.

Specify how to create a superuser account in dev

To do development you will need a local super user account. This is
a standard Django practice but should be documented here as well.

Add fixture data to the README

The site requires you to have some basic data installed in your database
before running. This is located in the fixtures file and so the README
should tell people to install this data.

Update the README

Attempting to reflect the current state.

Pycon seems to be using fixtures to setup the data that
populate_inventory does. Need to look into that.