Post #1015 task.

[Carreau]

Once #1015 is done, there are many remaining tasks.

1. LXML parsing fails on newer nbconvert when running notebooks that have SVG included.
2. slides do not hide the menubar.
3. nbconvert should have finer grained templates without headers.
4. security : gh api v3 have changed, one test is skipped.
5. Mock gh requests when testing, for speed and consistency. This also could let us check that the reply don't change.

[Carreau]

See jupyter/nbconvert#1894 as well.