Update custom.js

Explain different semantics required for instances versus functions or classes. Based on comments of @minrk in #5985.
Implement atomic save

Ping @fperez, this should avoid issues with corrupted/lost notebooks
when the disk is full, though I haven't worked out how to test it just
yet.

Closes gh-6254
Rework atomic_writing with tests & docstring
Update custom.js
Add fsync() call in atomic_writing
Load rpy2.ipython for rmagic, print deprecation warning
Remove rmagic tests
Remove rmagic docs

Leaving intersphinx link to rpy2 docs.
No need to check for rpy2 in test machinery
Docs fixes for rmagic deprecation
Add URL to warning about rpy2
build css

looks like a recent PR didn't have a final render of the LESS
fix handling of no-checkpoint-defined

a few typos and missing handling of undefined checkpoint

regression introduced in #6205
minor fixes in dashboard tests

events and regular expressions weren't quite right, which could cause occasional test failures.
Merge pull request #6370 from minrk/handle-no-checkpoint

fix handling of no-checkpoint-defined
Merge pull request #6371 from minrk/dashboard-test-quickfix

minor fixes in dashboard tests
Merge pull request #6359 from minrk/build-css

build css
Rename `abort_queues` --> `_abort_queues`
Merge pull request #6374 from dhirschfeld/about_queues

Rename `abort_queues` --> `_abort_queues`
Make installation of native kernelspec explicit

Instead of creating a kernelspec directory matching the running
frontend, return its spec without writing to disk.

Adds a command 'ipython kernelspec install-self' to write a kernelspec
for the current version of Python.