Handle 'deletable' cell metadata
Add tests for undeletable cells
Make cell be undeletable ONLY when metadata is explicitly false
Fix tests
Interpret another error as missing symlink support on Windows

This occurs if the system and Python support symlinks, but the user does
not have permission to create them.
Hopefully fix Windows test failures with symlink
Merge pull request #6583 from takluyver/win-symlink-tsets

Windows symlink test fixes
support `%gui qt5` in kernels

same as terminal IPython in #6496
didn't need anything beyond register
support `%matplotlib qt5`

just needed the key, since `%gui qt5` already works
support %matplotlib nbagg

we don't need to do anything, just add keys to a couple of dictionaries
Merge pull request #6587 from minrk/mpl-qt5

support `%matplotlib qt5` and `%matplotlib nbagg`
Fix out of date docstring
use standard html
Merge pull request #6592 from Carreau/html

use standard html
note how to start the qtconsole

at the top of the qtconsole doc
Merge pull request #6593 from minrk/command-doc

note how to start the qtconsole
Removing useless help text from "-i" command line option.
Merge pull request #6597 from ngoldbaum/master

Removing useless help text from "-i" command line option.
Merge pull request #6488 from jhamrick/lock-cells

Proof of concept for preventing cell deletion via metadata
Shell command formatting.

Makes the command to start the qtconsole consistent with the rest of the
shell commands in this document.