Enable users to find any tag on a site easily

- All tags are available to filter by, using find-as-you-type autocompletion like the rest of the tags boxes, with JavaScript on.
- Without JavaScript, this doesn't appear at all as it defaults to the non-JS filter view.

- As discussed, limit the number of tags shown in this view to be ten, to make the 'Filter tags' textbox more visible and to make things look tidier.
- Only show the filter box if there are more than ten tags on a site.

![screen shot 2014-07-16 at 16 54 32](https://cloud.githubusercontent.com/assets/355033/3601443/863e81ca-0d01-11e4-9b65-9c4ae1d19ec2.png)

I'm assuming that we are. This is the same limit as in https://github.com/alphagov/transition/blob/master/app/views/shared/_tag_list_field.html.erb.