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
Allow timeout and click callback
Add documentation to NotificationWidget methods
Add documentation to notification area
Small changes to notification widget
Add tests for notification area and widgets
Clarify stickiness of warning/danger notifications
Fixed off by one error in get_prev_cell

Not sure why this was a TODO. Maybe `find_cell_index()` returned zero at one time, but in the browsers I tested, it always returns null if not found.
Fixes #6561 instructions for installing dependencies on zsh/tcsh
Don't catch repr-errors in pretty

allow exceptions to be raised and displayed by the caller
show tracebacks for errors in formatters

instead of FormatterWarnings
handle missing 'text/plain' in displayhook

avoids KeyError when repr fails
Merge pull request #6569 from gcporras/fix_instructions_for_installing_project_dependencies

Fixes #6561 instructions for installing dependencies on zsh/tcsh
Merge pull request #6394 from minrk/repr-fail-traceback

show traceback for broken reprs
Update documentation for functions that can return null
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
Merge pull request #6565 from dsblank/patch-1

Fixed off by one error in get_prev_cell