there seem to be "only" 2 places where this make sens, the other
being href in mdcell, detach pager, help link in new windows.
set IPython._target = '_self' for try.jupyter.org shoudl work in
custom.js
instead of GET terminals/new
to be consistent with creating new notebooks.
We had to stop using GET notebooks/new
because browsers would create new notebooks when making preview thumbnails for commonly visited pages, etc.
I assume the same issue would apply to terminals