Merge pull request #5158 from ivanov/fix-5157

log refusal to serve hidden directories
Make sure we are in command mode before we select a new cell.
security.js docstrings
mention that Trust triggers reload
allow highlighting language to be set from notebook metadata
make trust notebook dialog a single paragraph
Merge Security Pull Request: google-caja

Adds HTML sanitization.

The basics:

- untrusted HTML is always sanitized, with no warning
  (there is console logging for changes made)
- markdown is always treated as untrusted
- no warnings for simply excluded output (e.g. Javascript)
- CSS tags and attributes are always stripped from untrusted HTML
- never check whether HTML is "safe," only sanitize
- add 'Trust notebook' to File menu
fix vertical position of command mode tooltip
horizontally center menubar popup
rewording according to feedback in #4802
proper styling for the tour
tidy up capitalization
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.