Add support for different kernel specs to kernels REST API
Add kernel name to sessions REST API

Also, some refactoring so that the relationship between session, kernels
and notebooks is managed in the SessionManager, not in the HTTP
handlers.
Update JS for kernels and sessions APIs
Delete session rather than killing kernel in test cleanup
Fix JS syntax
Solving #5997: instances of WidgetView (that are not instances of DOMWidgetView) do not trigger widget_area.show anymore.
Add default_kernel_name class level variable
Merge pull request #6026 from takluyver/kernelspec-rest-launching

Kernelspecs in REST API for kernels and sessions
Merge pull request #6115 from SylvainCorlay/widget-area-show

Instances of WidgetView do not trigger widget_area.show anymore.
Embrace flexible box model