Use the ideas from fofr's prototype to filter the tag dropdown list

- Use the existing tag list autocomplete functionality (used when we're
  adding tags on a mapping) to autocomplete this text box.
- Leave a comment about having still to work out a way to submit the
  values in the textbox as ?tagged=X querystring, so I don't forget.
Use an existing CSS class instead of inline styles for a list
Limit the number of tags shown to ten

- The default of fifty was getting unwieldy and made it harder to see
  the filter box.
Only show the filter box if there are over ten tags on a site
Submit the data in the textbox as the ?tagged= querystring value

- In order to make the querystring parameter be named ?tagged=, I had
  to name the textbox ID that too, for reasons I don't know.