Set default plugin scroll behavior for Firefox and IE11

[kellyi]

#808 reported a bug whereby the launchpad plugin wouldn't scroll in Firefox and IE11. #822 fixed the issue specifically for that plugin, but we should update the framework generally so ensure that plugins scroll in Firefox and IE11 if necessary by default.

The issue in #808 was that the launchpad didn't have a height rule set in its CSS. I think updating the framework might encompass setting some kind of default height for plugins when they've otherwise got it unset in their stylesheets.