Fix broken tag autocomplete on multiple pages.

Mark all tag <input>s with a `data-autocomplete` attribute, instead of
hardcoding a list of html IDs to autocomplete in javascript.

This way should be less error prone. It fixes autocomplete in several places:

* Autocomplete for the search box on /posts didn't work in the
  responsive layout. This was because /posts has two search boxes that
  both have the id `tags`: one in the normal sidebar, and one in the
  responsive tag list. $("#tags") only initialized autocomplete on the
  first one.

* Autocomplete didn't work on the aliases or implications pages. This
  was due to selecting the wrong html ids.

app/views/uploads/new.html.erb

@@ -101,7 +101,7 @@
         <div class="input">
           <div>
             <%= f.label :tag_string, "Tags" %>
-            <%= f.text_area :tag_string, :size => "60x5" %>
+            <%= f.text_area :tag_string, :size => "60x5", :data => { :autocomplete => "tag-edit" } %>
             <span id="open-edit-dialog" class="ui-icon ui-icon-arrow-1-ne" title="detach" style="display: none;"/>
           </div>