Remove single alias/implication requests.

* Remove the single alias and implication request forms. From now
  on, bulk update requests are the only way to request aliases or
  implications.

* Remove the forum topic ID field from the bulk update request form.
  Instead, to attach a BUR to an existing topic you go to the topic then
  you click "Request alias/implication" at the top of the page.

* Update the bulk update request form to give better examples for the
  script format and to explain the difference between aliases and
  implications.

app/views/bulk_update_requests/new.html.erb

@@ -1,5 +1,5 @@
 <div id="c-bulk-update-requests">
-  <div id="a-new">
+  <div id="a-new" class="fixed-width-container">
     <h1>New Bulk Update Request</h1>
 
     <%= render "form" %>