Add FK from asset to organization (an asset is owned by one organization)

### Dependies
- #47

### Overview
As a user I need to know organization ownership of the assets.

### Action Items
- [ ] Update models.py. See other examples of foreign keys in the file
- [ ] Run source ./start-dev.sh. This will run makemigration which will create a new script and run it

### How to Manually Verify
- [ ] From asset screen verify you can associate an asset category with an asset.

### Automated Tests
Automated tests will be done as part of another issue.

Start with #47, then inspect models.py for existing foreign-key examples and run source ./start-dev.sh to exercise migration creation. Verify the asset screen can associate an organization with an asset; the issue's automated tests are explicitly deferred to another issue.

Scritto dal modello di indicizzazione a partire dal testo della issue.