Implement shared assets in openedx_content [FC-0138] #890

Following the ADRs in #822, implement the "Asset" model in openedx-content, along with a suite of tests.

This should include tests related to asset dependencies, but does not need to include any platform changes, nor any support for /static/ references/detection. (In other words, asset dependencies work if manually created, but we don't create them automatically.)

Private ref FAL-4427