Make sure comm-dead is applied to the top widget view class
in all cases where the kernel disconnects.
Bubble event and implement logic in CodeCell
invoke css
fix restore checkpoint add an empty cell
make method private
Merge pull request #7247 from Carreau/fix-restore-checkpoint

fix restore checkpoint add an empty cell
Address comments
invoke css
Merge pull request #7227 from jdfreder/undead

Add visual cue for widgets with dead comms
allow ContentsManager methods to return Futures

mainly adding `@gen.coroutine` and `yield gen.maybe_future` all over the place.

No FileContentsManager methods are actually async at this point.
use gen.Return for Python 2
Fix HEAD requests for kernelspec resources

Closes gh-7237
Closes gh-7258

StaticFileHandler.get() is a coroutine. When Tornado calls a handler
method, it uses the return value to determine whether or not it's a
coroutine. So when head() calls get(), it needs to pass the return value
on for Tornado to handle it properly.
Merge pull request #7253 from minrk/async-contents-handlers

allow ContentsManager methods to return Futures
Merge pull request #7259 from takluyver/kernelspec-head-error

Fix HEAD requests for kernelspec resources