Use symbols, not strings, for component object keys

We should only use one style of keys for objects used in components,
to avoid reduce confusion when using components. Consistency is good.

After discussion with @edds we agreed to use symbols as this makes
more sense in a ruby/rails environment.

The first step was to support both, as clients of those components
will still be sending string key'd objects and can't be updated
at the time as this app, so we have to;

1. Support both kinds of keys on components that expect strings
2. Update clients sending string keys to use symbols instead
3. Remove support for string keys from those components

This PR is step 2. for `previous_and_next_navigation`, but also
needs doing in `finder-frontend` for `options_select`.

Step 1 is: alphagov/static#601

app/helpers/application_helper.rb

@@ -16,17 +16,17 @@ def previous_and_next_links(document)
     if document.previous_sibling
       siblings.merge!( 
         previous_page: {
-          "title" => "Previous page",
-          "url" => document.previous_sibling.base_path
+          title: "Previous page",
+          url: document.previous_sibling.base_path
         }
       )
     end
 
     if document.next_sibling
       siblings.merge!(
         next_page: {
-          "title" => "Next page",
-          "url" => document.next_sibling.base_path
+          title: "Next page",
+          url: document.next_sibling.base_path
         }
       )
     end