Better padding for spoiler text

With no padding currently surrounding [spoiler] text, the white text
gets close enough to the white post background that it's a bit more
difficult to read than it reasonably could be. Pad spoiler text with 2px
to alleviate this difficulty.

Signed-off-by: David Celis <me@davidcel.is>

app/assets/stylesheets/desktop/topic.css.scss

@@ -171,6 +171,7 @@
   span.spoiler {
     background-color: $black;
     color: $black;
+    padding: 2px;
     @include hover {
       color: $white;
     }