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/helpers/bulk_update_requests_helper.rb

@@ -1,4 +1,17 @@
 module BulkUpdateRequestsHelper
+  def bur_script_example
+    <<~EOS
+      create alias kitty -> cat
+      remove alias kitty -> cat
+
+      create implication cat -> animal
+      remove implication cat -> animal
+
+      mass update kitty -> cat
+      category touhou -> copyright
+    EOS
+  end
+
   def approved?(command, antecedent, consequent)
     return false unless CurrentUser.is_moderator?