Small design tweaks to the group share page

- Put a <br> between the document link text and the document hostname
  (if there is one)
- Double the amount of vertical space between each <li> in the list of
  documents
- Increase the number of characters after which document link text or
  hostname is truncated

h/static/styles/group-form.scss

@@ -81,7 +81,7 @@
   }
 
   .group-form-document-list li {
-    margin-top: .5em;
+    margin-top: 1em;
   }
 
   .group-form-footer {