allow --help and config_file classes to differ

override help_classes or config_classes
to allow them to differ from App.classes
only include kernel config in `--help` output

don't write it to a config file that won't be read
ipython_kernel gets its own config file
remove references to kernel config in parent config files

since it doesn't work anymore.
Link to the kernel config doc instead.
Merge pull request #6536 from minrk/parent-config

Remove kernel config from parent config files