Make checkboxes not use interior mutability

Currently `CheckBoxId`s are implemented like this:
```rust
#[derive(Clone, Default, Debug, Eq, PartialEq)]
pub struct CheckboxId(Arc<OnceCell<u64>>);
```
This should be changed to:
```
#[derive(Clone, Default, Debug, Eq, PartialEq)]
pub struct CheckboxId(u64);
```