Fix hover hit regions

Because the top parent had the padding for the icons, the specific elements'
:hover was not triggered when hovering the whitespace reserved for the icons.
This made the icons only appear when actually hovering over the icons' space,
or hovering over its container element on the left. Moving your mouse from the
left container to the right icon flashed shortly, because you left the hit
region.
While I was adding classes to the specific elements that need the right padding,
I figured I could improve the hiding/displaying of the icons. Instead of having
to make sure you show/hide the icon correctly per icon, the .flow-icon CSS now
takes care of it.

Meanwhile also improved the CSS for the input-like textarea to submit a reply.

Meanwhile also fixed some paddings related to post: reserve space for reply &
thank using visibility: hidden; fix some padding around the title

Change-Id: I64c2569f2bd3aa092d57bec27b672d9528f925b7