Forward port has_model to the 8.x widget manager

[jasongrout]

In #3377, I introduce has_model in the base widget manager class, simplifying logic.

Right now we have a confusing get_model method:

• In the base widget manager, it returns undefined if the model is not registered, which means that it can be used to synchronously check if a model is registered. This means the function is not purely async.
• In the jlab widget manager, the function returns a rejected promise if the model is not registered, making the function a truly async function, but leaving us without a way to easily check synchronously if a model is registered.

Once we have has_model by porting #3377 to master, I think we can change the base get_model to be a truly async function like it is in the jlab manager now. This is a breaking change, so would need to be in widgets 8.0.