css: ensure dtext is always wrapped in .prose container.

Ensure dtext is always wrapped in a `<div class="prose">` or
`<span class="prose">` (for inline dtext) container so that dtext css is
properly applied.

app/views/bans/index.html.erb

@@ -28,7 +28,11 @@
             </td>
             <td><%= time_ago_in_words_tagged(ban.created_at) %></td>
             <td><%= humanized_duration(ban.created_at, ban.expires_at) %></td>
-            <td class="col-expand"><%= format_text ban.reason %></td>
+            <td class="col-expand">
+              <div class="prose">
+                <%= format_text ban.reason %>
+              </div>
+            </td>
             <td>
               <% if CurrentUser.is_moderator? %>
                 <%= link_to "Edit", edit_ban_path(ban) %>