sideBySide: limit height of popover
sideBySide: limit height of popover

The popover seems to be ignoring the screen height
and grows indefinitely, this causes two issues:

a) the popover aesthetic is completely broken, and
b) the scrolled window never activates, so we can't
   display all the categories.

Set a maximum content height for the scrolled window
to workaround these 2 issues.

https://phabricator.endlessm.com/T19304

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib