comment design

This implements a new comment design. Most important changes:

-   all (sub)comments are visible by default
-   the show_comments button is obsolete
-   the new_comment button gets the same arrow as showhide_buttons
-   lines on the left make it simpler to understand the tree structure
-   the metadata is reordered
-   new comment forms don't have their own heading

The last commit does not really belong here but as it depends on these changes and does only minor css modifications I left it.