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.
Add a Cucumber feature to test the new search to filter functionality
Fix the alignment of the button - make it appear next to the input
Fix odd tag indentation by remove list wrapper

* Create a tag list as per the style guide
Use a form with hidden fields to submit tags

* Avoid using inline javascript to submit form
* Based on pattern used by new_url form
* Pre-fill tag filter form with any tags already being filtered by
Add tag field label
Refer to tags as "All tags" in filter field
Style select2 correctly when in an input-group

* Bootstrap 3 styles don’t reset the rounded corners of the select2
list element
Merge pull request #343 from alphagov/allow_filtering_tag_list

Enable users to find any tag on a site easily