factor valid cell index logic
refactor cellc
Updating notebook compat docs.
abstract, cleanup and document

Separate the methods that actually insert dom element for easier
testing.

Cleanup, and order methods more logically

add "docstring"
Removing old dev docs.  All of this has been moved to GH wiki.
Adding link from Sphinx dev docs to GH wiki Dev: Index.
simplify logic
remove unused/not working function
move CM IPython theme out of codemirror folder
added assert msg to extract_hist_ranges()
fix notebook deletion.

do not thrust what "that" is in a 3 level deep
callback
fix i/index in move up/down and == -> ===
fix one more == to ===
Changes from linting github tools
joined two message lines to one
Merge pull request #2966 from joonro/history-extract-assert-msg

Added assert msg to extract_hist_ranges()
Merge pull request #2968 from Carreau/fixdel

fix notebook deletion.
Merge pull request #2933 from ellisonbg/minordocupdate

Minordocupdate
Merge pull request #2929 from Carreau/cleanup-cell-insertion

Cleanup cell insertion
Merge pull request #2942 from Carreau/cm-ipython-css

Move CM IPython theme out of codemirror folder