Update fixtures

Remove fixtures from 2018 and 2019. Add fixtures for 2020.
Cleanup structure of fixtures directory. Update scripts to match.

Add a test fixture which creates a user for testing the CFP

Also add a single proposal which can be reviewed.

Update dev instance deploy script

- We have a flatpages fixture to add in
- Need to let the script exist in the docker context so it can be run

Don't populate inventory yet

We'll turn this back on once we're ready to start testing inventory for LCA2019

Update testing fixtures to add admin users

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

Build an ephemeral dev container for rego testing

Create a ephemeral dev container to facilitate testing of
populate_inventory. It can be created, tweeked in the admin interface,
files updated in admin's populate_inventory, then redeployed to deal
with those changes.

A branch based one will need to be created to so we can do this work
without polluting the master branch.