Make default kernel configurable

- The default value is stored in the MKM.default_kernel_name configurable.
- If `kernel_name` is left unspecified or specified as None,
  this value will be used, instead of hardcoding 'python'.
- A POST request to `/kernelspecs/:name` sets `name` as the default kernel.
- selecting a kernel from the menu sets the default kernel for new notebooks.

We will probably want to move where that POST request is called when we get the new-notebook-with-kernel dropdown menu, but I think everything else will remain the same.