qtconsole: Add vi-like key bindings to pager (fixes #6590)

Since _event_fileter_page_keypress() already exists in ConsoleWidget
class of qtconsole, that change was pretty easy and isolated. We emulate
up/down arrow keys with k/j when we use pager.
pin less to less than 1.7.5

1.7.5 drop all vendor-prefixed values that are set to the default value
of most browser leading to huge change in the number of lines in css and
pain everywhere.
Clean up session class
Fix reference to session in notebook.js
Fix bugs introduced into session.js
Clean up kernel.js
Fix messages received by notification area
User correct check for whether kernel is running in tests
Fix session references in toolbar and menubar
Fix bugs in kernel.js
Fix shutdown test
Better respect for abstraction barriers
Move channels into their own object
Only set channels to null when they are fully closed
Fix kernel and session tests
Update session tests
Add some more kernel tests
Handle query string in Kernel.start
More informative error messages
Add no_kernel.Kernel event