Enable line breaks in performatted text within rich text

This stops redaction details being truncated at the edge of the
page when they have long lines of preformatted text.

Also use a div instead of a paragraph for redaction details so
that formatting is consistent with other rich text blocks.

Closes #1885
Fixes #1845

app/assets/stylesheets/common.scss

@@ -2314,6 +2314,7 @@ a.button {
     font-size: 13px;
     background: #e8e8e8;
     padding: 2px 3px;
+    white-space: pre-wrap;
 
     code {
       padding: 0;

app/views/redactions/show.html.erb

@@ -7,10 +7,10 @@
   <b><%= t '.user' %></b>
   <%= link_to(@redaction.user.display_name, user_path(@redaction.user)) %>
 </p>
-<p class="richtext">
+<div class="richtext">
   <b><%= t '.description' %></b>
   <%= @redaction.description.to_html %>
-</p>
+</div>
 
 <% if current_user and current_user.moderator? %>
 <div class="buttons">