Center the Join Button in Contest List

resources/widgets.scss

@@ -100,6 +100,12 @@
             background: darkblue;
         }
     }
+
+    // class = "unselectable button full small" only appear in online-judge/templates/contest/list.html
+    // this attribute center buttons in contest list (including "Join", "Virutal Join", "Spectable")
+    &.unselectable.button.full.small {
+        margin: 0 auto
+    }
 }
 
 .button.full, input[type=submit].full, button.full {
@@ -590,4 +596,4 @@ ul.select2-selection__rendered {
 
 .control-button:hover {
     background: gray;
-}
+}