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
Remove a redundant test
Change describe blocks to context blocks in BulkAddBatch spec
Remove `before {}` noise by using one in a new `invalid` context
Merge pull request #350 from alphagov/tidy_up_bulk_add_batch_specs

Tidy up the BulkAddBatch spec
Remove unused methods from SiteYamlFile
Merge pull request #352 from alphagov/remove_unused_methods

Remove unused methods from SiteYamlFile
Move MappingsProgress into a controller concerns directory

This is the idiomatic place for concerns to live (especially as of
Rails 4) so seems sensible, even though we're not using ActiveSupport::Concern
which would generally be used by concerns which live here.

Having this code in app/ rather than lib/ also means that the dev server
reloads it for us.
Move CheckUserCanEditMappings into app/controllers/concerns/