refactor comment css

There are several issues with the comment styling:

*  there is no spacing at the bottom of list items
*  deprecated (and broken) arrows are used in buttons as #470 was never merged for different reasons
*  lists inside comments are broken
*  the padding of new_comment and show_comments changed when hovering because the button styling was only applied in `:hover` state
*  vote, new_comment and show_comments are not vertically aligned as we can not know their sizes in advance. e.g. in some themes round buttons (and therefore the vote component) have 1px borders while small buttons have not resulting in a 2px difference in height.

I fixed all these issues except for the last one.

![2013-07-23-115942_1440x900_scrot](https://f.cloud.github.com/assets/202576/840959/db69970a-f382-11e2-868b-35161f3c0bbc.png)