Check version control status and common Django errors

[vsalvino]

To help eliminate common foot-guns, and improve the developer experience, we are adding a set of new checks to help prevent accidental deployment errors.

On the check, deploy, and upload commands, run a set of new "predeploy" checks which includes:

• Check if the git branch matches the deployment environment (staging/prod).
• Check if there are un-committed or un-pushed changes.
• Run manage.py check
• Run manage.py makemigrations --check.

These can be disabled by using the --skip-predeploy argument.

Additionally, in non-interactive environments (i.e. CI/CD pipelines), cr will automatically continue or cancel based on the severity of the check that has failed.

Fixes #12