Fix django warning, DB migrations, and integrity error

cryton/cryton_app/migrations/0001_initial.py

cryton/cryton_app/models.py

cryton/lib/models/step.py

examples/advanced-example/execution-variables.yml

examples/advanced-example/inventory.yml

examples/advanced-example/report.yml

examples/advanced-example/template.yml

examples/basic-example/inventory.yml

examples/basic-example/report.yml

examples/basic-example/template.yml

examples/datetime-trigger-example/report.yml

examples/datetime-trigger-example/template.yml

examples/empire-agents-example/inventory.yml

examples/empire-agents-example/report.yml

examples/empire-agents-example/template.yml

examples/execution-variables-example/execution-variables.yml

examples/execution-variables-example/inventory.yml

examples/execution-variables-example/report.yml

examples/execution-variables-example/template.yml

examples/http-trigger-example/report.yml