Fix #4279: Very long tags break forum post format.

* Enable word breaks for all DText-formatted text. Ensures extremely
  long strings in comments get broken.

* Fix the author column on comments/forum posts to use a fixed 12em
  width (except on the mobile layout). Using flex-basis was wrong because
  it allowed the author column to shrink if the other column was too wide.

app/javascript/src/styles/common/dtext.scss

@@ -2,6 +2,7 @@
 
 div.prose {
   line-height: 1.4em;
+  word-break: break-word;
 
   h1, h2, h3 {
     line-height: 1em;