Add a block/unblock button to the infobar

This adds an easier way of blocking individual sites. No need to
switch to the list view and perform the action there, it can all
be done within the graph view now.
Use color buttons to match the list tab

Replaced the monochrome button icon (styled after the left sidebar
buttons) with a the color one that's used in the list view.

Also added a green version of the icon since it looked weird to
have a red "forbidden" symbol next to "Unblock Site".
Merge pull request #544 from fmarier/infobar_blocking

Add a block/unblock button to the infobar