add utils.get_url_param

I really cannot believe that we need to write this ourselves
allow kernel_name to be specified in url parameter
change kernel spec placeholder name to 'Kernel'
handle undefined settings in promising_ajax
Add kernel-select dropdown to new notebook button

most recent choice is remembered in localStorage
include current kernel name in new notebook dropdown
separate selecting default kernel from new notebook with specified kernel

adds checkmark to indicate selection
kernelspecs is a dict

sort menus client-side
store current kernel selection in frontend config

instead of localStorage
remove UI for setting default kernel

can be set with:

    IPython.new_notebook_widget.set_default_kernel('kernel_name')
remove some weird tabs

How did they get there? We may never know...
informative hover text for current selection
minor reflow of tree header

- remove absolutely positioned text (absolute text doesn't wrap correctly)
- associate hidden upload directly with 'click here' via span
- remove column split, rely on regular reflow
s/current_selection/default_kernel/
Merge pull request #6950 from minrk/new-new-dropdown

Add kernel-select dropdown to new notebook button