bans: change expires_at field to duration.

Changes:

* Change the `expires_at` field to `duration`.
* Make moderators choose from a fixed set of standard ban lengths,
  instead of allowing arbitrary ban lengths.
* List `duration` in seconds in the /bans.json API.
* Dump bans to BigQuery.

Note that some old bans have a negative duration. This is because their
expiration date was before their creation date, which is because in 2013
bans were migrated to Danbooru 2 and the original ban creation dates
were lost.

app/views/bans/edit.html.erb

@@ -2,11 +2,7 @@
   <div id="a-edit">
     <h1>Edit Ban</h1>
 
-    <%= edit_form_for(@ban) do |f| %>
-      <%= f.input :duration, :hint => "in days" %>
-      <%= f.input :reason %>
-      <%= f.button :submit, :value => "Ban" %>
-    <% end %>
+    <%= render "form", ban: @ban %>
   </div>
 </div>