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.
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.
Fix broken show in pager button
Merge pull request #6673 from jhamrick/fix-tooltip

Fix broken show in pager button
Have /api return the IPython version

return version as a string, to be consistent with other APIs.

closes #6647
strip cell.trusted as a transient value

We write trust to cell.metadata.trusted now,
but used to write it to cell.trusted.
Just make sure we never get either from disk.
Merge pull request #6654 from Carreau/closes-6647

Have /api return the IPython version