Change spacebar to work like browser default

[jnunemaker]

I often use spacebar to navigate down a "page" at a time in the browser, but it doesn't work that way in the pane that shows an individual notification. When a notification is showing that can be scrolled, it seems the browser default of hitting space and advancing down the scrollable area makes more sense.

[bkeepers]

:+1:

[sheriferson]

I was about to leave feedback about this. Being able to use spacebar to scroll the right pane would be the last piece in making one able to view all notifications without having to use the trackpad or mouse at all.

But I see that this issue was opened back in 2014, and the last commit was over a year ago, so my guess is that it won't happen - but would be great if it did.