put Running tab on the page
minimal KernelList
ok, Running tab is working now
fix typo in method name
refresh of Notebook list should reload sessions
don't double-join the notebook path
remove reiterated repetitively redundant "actively"
added IPython.session_list

before this, kernel_list and notebook_list each fetched and held onto
their own copy of the sessions.
refer to notebooks, not kernels.
use explicit running header name + jquery's toggle
remove redundant checks in code
small whitespace cleanup, renamed drag_info

in the dashboard, I've renamed drag_info to notebook_list_info, so
applying style to notebook_list_info and running_list_info can be done
in one place.
added Running tab note to the docs
Merge pull request #5215 from ivanov/running-kernels

Dashboard "Running" Tab