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
a few 1 pixel fixes.

this uses the same html/css structure acroos the tabs, and reduce the overlay
click zone that expended the size of the header to 25px instead of 24px
BF(BK): cleanup tempdir in EmbeddedSphinxShell atexit
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.
Merge pull request #7339 from minrk/spec.display_name

update new notebook button with kernelspec changes
Update Dockerfile

Latest master removed the `--system` flag. You can pass a `--user` flag for non-system install.
Merge pull request #7302 from Carreau/1pix

a few 1 pixel fixes.
Display kernels in alphabetical order.
Merge pull request #7345 from jdfreder/kernel-order

Display kernels in alphabetical order.
Merge pull request #7343 from dsblank/patch-2

Update Dockerfile
Fix invalid JSON

Trailing comma in last property causes the JSON file to be invalid.
use 'Python X' for IPython kernel display name

instead of 'IPython (Python X)'
update kernelspec test

to expect Python display name