web: Remove screenshots, license and credits page.

The server and audiod log files provided little information and
were constantly outdated, so this patch removes them. The para_gui
screenshot is turned into a link in the feature list where para_gui
is mentioned. This gets rid of the screenshots page.

The paraslash license is mentioned on the main page, so we don't
need a dedicated page for it. The CREDITS file stays in the git repo
of course but we don't need a web page for this either. It's fairly
static anyway.