Don't make the notifications list 5px taller than the screen

The max-height of the notifications list was hardcoded to
the height of the window minus 134 pixels. I don't know
where this value came from, but it doesn't seem to take
borders into account. The correct value appears to be
139 in Chrome and 140 in Firefox, so I changed it to 140.

Also, this really really should not be a hardcoded value.
This 140 value can be measured in JavaScript; that's how I
derived it. Even better, oojs-ui has a mixin called
OO.ui.ClippableElement that automatically does this for you
AND automatically adjusts the height when the window is resized
as well.

Change-Id: I17bc2c5333e5c3d5dd720e6bccf8cbbdbb4abe6c
Merge "Don't make the notifications list 5px taller than the screen"