add resource URLs to kernelspec model

- move kernelspec itself to model.spec
- add resources top-level key with resource URLs
- no more 404s or HEAD requests under normal circumstances
update kernelspec model tests
remove whitespace below menubar when toolbar hidden.

closes #7292
reformat css

cause I want to inject some variable in it after (like border radius)
and a few other thigs.

This is just a reformating that does not change the generated css.
MAINT: Move `update` from FileContentsManager to ContentsManager.

The implementation in FileContentsManager is written entirely in terms
of the required API, and is sensible for other ContentsManager
implementations.
Merge pull request #7308 from Carreau/reform-css

reformat css
Merge pull request #7312 from quantopian/refactor-contents-update

MAINT: Move `update` from FileContentsManager to ContentsManager.
fix border radius on element.

set a value to border-radius large, and actually make border radius
small smaller than border radius large.

Make dialog border radius coherent with the rest of the UI.
unify submenu style with menu style

closes #7303.

Also change the arrow that indicate submenu for a fontawesoem icon,
instead of the ugly hack that show only the left border of a rectangle element
which with is twice the one of its border to get a triangle.
stack vertically  keyboard shortcut on narrow viewport.
BF(BK): cleanup tempdir in EmbeddedSphinxShell atexit
Merge pull request #7320 from Carreau/narrow-vp

stack vertically  keyboard shortcut on narrow viewport.
Merge pull request #7318 from Carreau/dialog-border-radius

fix border radius on element.
Merge pull request #7311 from Carreau/reform-css-2

Reform css 2
Merge pull request #7307 from Carreau/remove-white

remove whitespace below menubar when toolbar hidden.
friendlier error messages when invoke/lessc are missing

and not needed.
Merge pull request #7316 from yarikoptic/enh/emb_sphinx_shell_cleanup

BF(BK): cleanup tempdir in EmbeddedSphinxShell atexit
Merge pull request #7326 from minrk/less-invoke-missing-messages

friendlier error messages when invoke/lessc are missing
Merge pull request #7283 from minrk/kernel-spec-model

add resource URLs to kernelspec model
update new notebook button with kernelspec changes

display_name is in the spec, not top-level.