remove cython extension.

Now in cython package itself, as stable.
remove Cython Built-in extension notebook
remove more ref to cythonmagic
use Cython.load_ipython_ext to simplify
add whatsnew and uppercase C in load_ext

Use uppercase as this is what is in cython docs.
Make comm manager (mostly) independent of InteractiveShell

This makes it possible to use comms from wrapper kernels, without
instantiating the full IPython shell machinery.

The one remaining place where we need a reference to shell is to fire
pre_execute and post_execute hooks (which are needed to get mpl figures
right). This is a pure IPythonism, that it should be safe to ignore if
shell is not set.
Make comm_manager a property of kernel, not shell
Safeguard against comm instantiation with both session and kernel (corner case)
qtconsole: strip newline on text copy (fixes #6234)

IPython's inputsplitter inserts newline at the end of string passed to
it. So, if user didn't select newline before copying, we must strip it.
Fix code_to_line when called with empty code

Previously, it was referring to line before it was assigned. Noticed
when testing the R kernel.
Separate widget model name from com target name.
Generalise render/unrender to all cells type

Allow change of behavior in pure (custom) css.
Fix grammar in comment
s/target_name/model_name
Merge pull request #6665 from jdfreder/travis_please_run_on_this

Separate widget model name from com target name.
Merge pull request #6657 from takluyver/adapt-no-code-to-line

Fix code_to_line when called with empty code
Merge pull request #6651 from JIghtuse/strip_newline

qtconsole: strip newline on text copy (fixes #6234)
Merge pull request #6416 from Carreau/foobarcode

Unify Text/CodeCell code.
remove on_first_message authentication

in ZMQStreams. No need for weird, special first message.

- use regular cookie auth
- use url param for session id
Fix broken show in pager button