Updates to item creation

[Carifio24]

This PR makes a few changes related to adding new students/educators/classes:

• Expect 201 response codes (rather than 200) for successful student & educator registration
• Update to use the newer /create endpoints (note that the old ones still work fine)
• Update class creation (in both the frontend and the backend) to account for the class expected size added in Add expected size to class cds-api#151

A couple notes:

• The expected size input field in the class creation dialog uses Solara's InputInt and thus the styling is somewhat different. I wasn't able to find a simple way to make the styling match the other two input widgets, but I'll come back to that in the future.
• The behavior of the InputInt is (IMO) a bit unintuitive. If the user enters something invalid, the associated value from the widget just stays as whatever it was before. So if I start with an expected class size of 20, then put in, say, "42.7", then the expected_size value is still 20, which will get passed to the database. Also, AFAICT, there's no way to access whether the widget input is in an error state, so I wasn't able to hook a check for that into the button-disable computed.

[Carifio24]

I've also now updated this to allow a teacher to set whether a class will be asynchronous, which was enabled via the API endpoint in cosmicds/cds-api#153.

[nmearl]

You could use the base rv.TextField and parse the integer manually. You can see that, under the hood, it's what solara.InputInt does already. That way you can enforce rules yourself and apply custom styling more easily.

[Carifio24]

@nmearl Yeah, that's probably the way to go, particularly for the error state stuff. I'll update that, and then the computed value that allows submitting can be more robust too.

[Carifio24]

@nmearl I did something similar to what you had done with the FigurePlotly in Hubble - I stole the Solara code and tweaked it to add the options that we wanted. The input field now matches visually and allows us to respond in the portal to whether the input is in an error state.