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
Only allow iframe embedding on same origin.
Incompatibility notice on iframe embedding
Merge pull request #6115 from SylvainCorlay/widget-area-show

Instances of WidgetView do not trigger widget_area.show anymore.
Merge pull request #6120 from rgbkrk/no_iframe_embedding

Only allow iframe embedding on same origin by default.
remove rackcdn https workaround for mathjax cdn

cdn.mathjax.org works over https now
Merge pull request #6134 from minrk/mathjax-cdn

remove rackcdn https workaround for mathjax cdn
return global events from require(['base/js/events']).

In master, it is impossible to load code on IPython events without setting timeouts.
don’t use events !== undefined to signal page ready

it will always be defined