Move git services to tuebingen.mpg.de.

Both the gitweb and the git daemon service have been relocated to
tuebingen.mpg.de. This commit changes the git-related URLs in the
repository to their counterparts at the new location.

The new location does not have a gitweb page dedicated to paraslash
alone. Instead, the gitweb service is provided by the gitolite
instance at git.tuebingen.mpg.de which hosts also a couple of other
repositories.

The "Changes" link of the the menu of the paraslash web pages points
to the dedicated gitweb page, so this link is removed from the menu.
There is still a gitweb link on the download page and a new one in
the README file.

Without a dedicated gitweb page, parts of the para.css style sheet
are no longer needed. This patch removes these parts, along with
git-logo.png, header and footer, which also have become unused.