Add a hover effect on left menu links (#418) (#425)

I:
- removed the `margin-left` on the `.panel-block` to put it on the children
instead
- squashed the `margin-bottom` of the links into their `height` (and rounded it
to 40px, feel free to correct me)
- centered vertically the content of the links
- added the `background-color` change on hover for the links

I put the same color as the `.logged-in-block`. Maybe it should be externalized
as a SCSS variable?

I also added a CSS transition. Is it alright?