feat: model for upstream-downstream links

[navinkarkera]

Adds models and api's for Publishable entity links.

See openedx/edx-platform#36111 for more information and testing instructions.

Related to openedx/modular-learning#243

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[pomegranited]

👍 I left a few nits/questions about some field types and naming conventions, but this is working well :)

• I tested this using the test instructions on feat: upstream-downstream entity linking edx-platform#36111
• I read through the code
• I checked for accessibility issues by using my keyboard to navigate N/A
• Includes documentation -- code comments for modules and classes
• User-facing strings are extracted for translation -- mostly, but I've noted where the model verbose_name fields need marking too.

[ChrisChV]

@ormsbee Pinging you here if you want to take a look 👀