Duplicate loading observables

Currently, the datatable contains a `loading` observable that is set before loading data from the server. There is also a `showLoading` computed which just returns the value of the `loading` observable.

We should eliminate the `showLoading` computed and just have the `loading` observable.

For now, we could just point `showLoading` to the `loading` observable for backwards compatibility and note that `showLoading` is deprecated.

Locate the datatable code that defines the `loading` observable and `showLoading` computed, then inspect how `showLoading` is consumed. Keep the existing loading behavior while preserving `showLoading` for backwards compatibility and marking it deprecated; verify that the observable remains usable by existing consumers.

Written by the indexing model from the issue text.