Scrub hardcoded /home/dissimulo paths; use $HOME / relative symlinks

- Convert 21 absolute-path symlinks into YADR to relative paths
  (e.g. .vimrc -> .yadr/vimrc), so they resolve correctly under
  any user's $HOME after a standard YADR install.
- .fehbg: replace hardcoded background path with $HOME-based one
- .gtk-bookmarks: remove (trivially personal; GTK regenerates)
- .themes/*/index.theme: drop BackgroundImage= lines pointing at
  personal image files that other users won't have.
- .xscreensaver: scrub username from header comment.
- .irssi/config: generic dcc_download_path.

.irssi/config

@@ -377,7 +377,7 @@ settings = {
   "irc/core" = { cmds_max_at_once = "999999"; cmd_queue_speed = "0ms"; };
 
   "irc/dcc" = {
-    dcc_download_path = "/media/dissimulo/PhantomPhreak/Anime";
+    dcc_download_path = "~/Downloads";
   };
   "perl/core/scripts" = {
     dccstat_short_mode = "no";