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.

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

Add devmode_settings.py for local dev

* Adds an env variable to .env to turn on dev mode
* Uses the inbuilt django auth rather than saml when in dev mode
* For laziness, re-uses the admin login form for the non-admin login

make_dev_container now comes up ready to be logged into by any
user. No non-admin users are preconfigured; but you can add one at
http://localhost:28000/admin/auth/user/add/, log out, and then hit
http://localhost:28000/ to log in as the new user.

Create a dev docker container for makemigrations

* Adds a laptop-mode-env file which docker can read env variables from
* Adds a Dockerfile.makemigrations; mostly identical to the main
Dockerfile. Important difference: instead of the source being copied
into the docker image at build time, it's mounted from the local
machine at run time.
* Adds a makemigrations shell script which builds an imagine using the
Dockefile.makemigrations and then uses it to run makemigrations
* Because the source is mounted from the local machine, any new
migrations created are dumped in the developer's git checkout ready
for adding to git.

Rename .env -> docker/laptop-mode-env