feat: add workflow app and test migrations

[iloveagent57]

Description:
Add a description of your changes here.

Jira:
ENT-10071

Instructions for how I generated the test migrations
I wanted to have real model tables for testing the abstract workflow models exist in the sqlite test DB, but not exist in the DB for any other environment. I achieved this as follows:

1. Create a file named models.py inside the workflow/tests directory.
2. workflow/tests/apps.py must exist, and specify the app_label enterprise_access.apps.workflow.tests.
3. Modify the test settings file to include enterprise_access.apps.workflow.tests in INSTALLED_APPS.
4. Run makemigrations against the test app to create the migration file: ./manage.py makemigrations tests --settings=enterprise_access.settings.test.
5. And then we can add a unit test which actually makes use of the concrete model TestWorkflow.

Merge checklist:

• ./manage.py makemigrations has been run
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance

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