prevent saving of partially loaded notebooks

closes #5363
move prompts from margin to main column on small screens

This is mainly for nbviewer, but the relevant CSS lives in IPython.
Separate get_session_info between HistoryAccessor and HistoryManager

HistoryAccessor has no concept of the current session, whereas
HistoryManager does. HistoryAccessor should therefore always be used
with a session number > 0. However, I have added get_last_session_id()
to HistoryAccessor to make it easier to work with recent history.

Closes gh-5348
add notebook security doc
add ipython_security public key
move notebook docs to source/notebook
fix now-confusing section name in public server doc
add redirects for notebook docs moved out of interactive
move PGP key to download link
Fix tooltip pager feature

Updates to tooltip.js for msg spec changes.
Update COPYING.txt

Don't exclude other projects

Added new line

Don't specify that the comment token in Javascript is different from Python.

Fix line break

Update 3-clause line
renamed:    COPYING.txt -> COPYING.rst
s/COPYING.txt/COPYING.rst
Merge pull request #5429 from jdfreder/tooltipcompleter

Fix open-in-pager from tooltip
Make sure `element` is correct in the context of displayed JS

closes #5293
address PR feedback
Merge pull request #5396 from ivanov/fix-5363

prevent saving of partially loaded notebooks
Merge pull request #5430 from jdfreder/elementappend

Make sure `element` is correct in the context of displayed JS
Merge pull request #5419 from minrk/responsive-prompts

move prompts from margin to main column on small screens
Merge Security Pull Request: security-doc

Document the notebook security and trust model.