Look for kernelspecs relative to sys.prefix

Another alternative to #6966 and #7124, after discussion with @minrk .

This version will look for kernelspecs in `{sys.prefix}/share/jupyter/kernels` (N.B. now jupyter instead of IPython), but it doesn't provide a mechanism to put kernelspecs there, which avoids the tricky decision of when that's the right thing to do. Like #7124, it makes system installation the default, and there's a flag for user installation.

TODO: What should Python based kernels (including our own as well as wrapper kernels) do about installing into environments? Possibly try a systemwide installation, and on a permission error try again relative to sys.prefix?