Added PDF and Serve post-processor tests
FIXED postprocessor test typos
minor checkpoint cleanup

- remember list of checkpoints browser-side
- cleanup references in MenuBar restore list.
  There was a closure issue, where multiple menu items would actually all restore the same checkpoint.
complete on % for magic in notebook

fixes #3954
fix completion when shared start among completion item is null
don't make sphinx a dependency for importing nbconvert
fix `--existing` with non-localhost IP

there's still some cleanup to do with duplicate code in ConsoleApp that should be invoked from ConnectionFileMixin, but this fixes the bug.
add debug statements for kernel clients connecting to channels
make tab visible in codemirror and light red background

Shoudl fix #3935
remove red background, cleanup less file
BF: fix nbconert rst input prompt spacing

I believe this fixes gh-3980
Merge pull request #3960 from minrk/nosphinx

Don't make sphinx a dependency for importing nbconvert
Merge pull request #3981 from matthew-brett/rst-nbconvert-fix

BF: fix nbconert rst input prompt spacing
add missing 'control' socket to ConsoleApp.load_connection_file
test some connection file utilities

includes regression test for #3977 and some basic coverage for a few utilities.
Merge pull request #3955 from Carreau/fix-3954

complete on % for magic in notebook
Merge pull request #3939 from minrk/checkpoints

minor checkpoint cleanup

remember list of checkpoints browser-side
don't clobber list when a new checkpoint is created
cleanup references in MenuBar restore list. There was a closure issue, where multiple menu items would actually all restore the same checkpoint.
remove unused imports
Merge pull request #5 from ivanov/existing_ip

remove unused imports
Merge pull request #3978 from minrk/existing_ip

fix `--existing` with non-localhost IP

closes #3977