allow highlighting language to be set from notebook metadata
Fix kernel restart in case connection file is deleted.

On Mac, the /tmp files are deleted after 3 days if inactivity,
causing qtconsole kernel restart to fail subsequently.
This commit adds check to ensure that connection_file is
written if it does not exist.
Merge pull request #5272 from minrk/highlight-language-metadata

allow highlighting language to be set from notebook metadata
Store timestamps for modules to autoreload

Closes gh-4127

At present, autoreload compares the timestamps on .py and .pyc files.
However, this can fail in a couple of ways (see gh-4127). With this
change, it caches the mtime of the .py file when we loaded it.

Some extra complexity is needed to store the mtime as soon as possible -
we scan sys.modules when autoreload is loaded, and register a
post_execute hook to check for newly imported modules after each cell.
Use new events system for post_execute callback
check that PIL can save JPEG to BytesIO

as far as I can tell, this is a bug in PIL that is fixed in Pillow.
Update whatsnew doc from pr files
add note about PIL's jpeg support
Merge pull request #5276 from pankajp/recreate-conn-file-if-deleted

Fix kernel restart in case connection file is deleted.
Merge pull request #5278 from takluyver/pr-whatsnew-4

Update whatsnew doc from pr files
Merge pull request #5044 from takluyver/autoreload-nopyc

Store timestamps for modules to autoreload
Merge pull request #5277 from minrk/check-jpeg-test

check that PIL can save JPEG to BytesIO
nbconvert: Make sure node is atleast version 0.9.12
Cleanup
Split logic onto multiple lines.
append Firefox overflow-x fix
Fab CSS
Merge pull request #5280 from jdfreder/ff_cmurlpref

append Firefox overflow-x fix
Only check for node on first call, not on import.
fix a typo introduced by a rebased PR

caused heading cells to appear in both rendered and unrendered state