refactor: change progress-xs to progress-thin

Author: mrholek

scss/_progress.scss

@@ -47,14 +47,10 @@
   }
 }
 
-.progress-xs {
+.progress-thin {
   height: 4px;
 }
 
-.progress-sm {
-  height: 8px;
-}
-
 // White progress bar
 .progress.progress-white {
   background-color: rgba(255, 255, 255, .2);