remove base_kernel_url

While we are simplifying base_project_url to base_url, base_kernel_url has never been documented, and has only ever been confusing. In every case I have seen, base_kernel_url should be identical to base_url, so this just removes the separate configuration value.

I would also like to consider getting rid of ws_url, unless someone can make a case for why it should be kept.

closes #5099