Added clickable icon to collapse pager

Closes #3196
changed ESC to trigger collapsing of the pager in notebook
Collapse -> Close in button description as suggested by @fperez
use alternate method to collapse pager from notebook.js as suggested by @Carreau
Merge pull request #3239 from eteq/easy-close-pager

Allow "x" icon and esc key to close pager in notebook