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.

Empieza con #47, luego revisa models.py en busca de ejemplos existentes de claves foráneas y ejecuta source ./start-dev.sh para probar la creación de migraciones. Verifica que la pantalla de assets pueda asociar una organización con un asset; las pruebas automatizadas del issue se posponen explícitamente para otro issue.

Escrito por el modelo de indexación a partir del texto del issue.