formatting: bulk remove trailing whitespaces

Do not touch vendorized files (e.g. libraries that were imported from external
projects).

No functional changes.

Command:
    find . -name '*.<EXTENSION>' -type f -print0 | xargs -0 sed -i 's/[[:space:]]*$//'

src/static/skins/no-skin/pad.css

@@ -1,5 +1,5 @@
 @media (max-width:600px) {
-  #sidediv { 
+  #sidediv {
     display: none !important;
   }
 }