feat: Allow IDs or Models to be passed to all publishing APIs

[kdmccormick]

Part of: #562

To reduce noise in the implementations, I'm proposing these general patterns:

• Keep the param positional so that callers don't care if it's model, model_id, or model_or_id.
• When the function just needs the ID, name the param model_id and immediately invoke model_id = get_model_id(model_id).
• When the function needs the model instance, name the param model and immediately invoke model = get_<model>(model).

If this looks good, I'll apply it to the other applets as well.

BREAKING CHANGE: Some API functions which previously accepted int now accept <Model>.ID. Runtime behavior has not changed, but callers may need to use additional annotations or typing.cast(...) in order to get their typechecker to pass.

[kdmccormick]

Yes, I know this is funny :P But it makes the rest of the code simpler.

[kdmccormick]

list is invariant (its items must be the exact type, not subclasses thereof), whereas Sequence is covariant, thus more likely to behave with mypy the way consumers expect.

[kdmccormick]

@bradenmacdonald Could you review? If you like it, I'll do the other applets this way too.

[bradenmacdonald]

@kdmccormick I like it!

Could you please add a test for the get_model_id() helper that uses assert_type to verify that it's returning a typed ID like LearningPackageID correctly? It is working perfectly already; I just want to ensure that it stays working and doesn't downgrade to returning typing.All in some future refactor.