Add bottom padding to topic full text

Prevents scrollbar from showing up when there is a
subscript on the last line of text.

Another option would have been overflow-y: hidden,
but that clips the text in the (pathological?) case
of deeply nested subscripts.

tildes/scss/modules/_topic.scss

@@ -360,6 +360,7 @@
   @extend %text-container;
 
   overflow: auto;
+  padding-bottom: 0.4rem;
 }
 
 .topic-comments-header {