Add about dialog in Notebook Help Menu.

This allow to get info on version of IPython when running remotely.
The about dialog also send a kernel info request and display the banner
which is useful for non-python kernel that don't match IPython version
make pretty.max_seq_length configurable

via PlainTextFormatter.max_seq_length
cache sys-info
debugging websocket connections

- add debug statement at the very beginning of a web socket request
- add debug statement in open, indicating that the connection has been accepted
- add timeout, so failed or slow kernel_info doesn't cause the request to never get a response
- don't send kernel_info_request before authenticating the request

The last one required some icky coroutine shenanigans,
because of our subclass structure, but it should work fine.
Update Dockerfile.

Fixed to not use `apt-get upgrade` &
`apt-get update` is not on a single line
re: dockerfile best practices
https://docs.docker.com/articles/dockerfile_best-practices/

Signed-off-by: Jessica Frazelle <jess@docker.com>
add system-wide IPython config

Mainly for use in multi-user administrated environments

- config files in `/usr[/local]/etc/ipython` will be loaded
- change priority config loading to load all found files in priority order
  instead of loading only the first found.
Merge pull request #6807 from minrk/debug-ws-open

debugging websocket connections
Merge pull request #6806 from minrk/pretty-truncate-config

make pretty.max_seq_length configurable
BUG: Set file encoding for IPython.html.terminal.handlers.
Merge pull request #6811 from jfrazelle/update-dockerfile

Update Dockerfile.
Merge pull request #6813 from quantopian/terminal-handler-encoding

BUG: Set file encoding for IPython.html.terminal.handlers.
Update whatsnew file from pull requests
Merge pull request #6816 from takluyver/pr-whatsnew-8

Update whatsnew file from pull requests
Merge pull request #6810 from minrk/system-wide-config

add system-wide IPython config
Add whatsnew entry for PR #6810
Merge pull request #6554 from Carreau/itsallaboutme

Add About IPython
fix Windows typo

introduced in #6810
Merge pull request #6819 from minrk/windows-typo

fix Windows typo
Initial interface for javascript contentmanagers

contentmanager.js is going to be a js proxy for the current
filenbmanager.py.  This will allow a contentmanager for Google Drive to
be created.
ContentManager function signatures updated