load the per kernel kernel.js and kernel.css

As per discussion, each kernel can provide a file name kernel.js that
we try to load at kernel switching. If such a file exist we assume that
the kernel pathches the javasscript and that this javascript cannot be
unpatched, and further switching of the kernel cannot be undone without
reloading the page. (separate PR for UI)

if a kernel provide kernel.js, the it should consist into a AMD module
definition that uses require.js the module shoudl define a function name
`onload` that will be called at the appropriate moment before the kernel
starts.
Fix typos, rephrase english. Thanks Kyle.
adresses Min comments
move setting nb.kernelselector inside kernelselector itself
Merge pull request #7088 from Carreau/kernel.js

Load the per kernel kernel.js and kernel.css