Tweaks to services and info pages

Various tweaks following first version of services and information page, based on comments.

Story: https://www.pivotaltracker.com/story/show/75342126

* Sort sub-sectors alphabetically by title;
* Handle empty services and information pages by 404;
* Add styling tweaks to "See more" links";
I am not sure this does quite what I would expect. The call to `organisation_has_services_and_information_content?` sets the `@collection_groups` instance variable. So the call to  `collection_groups` doesn't actually do anything. This could be rewritten as:

```ruby
unless organisation_has_services_and_information_content?
  render status: 404
end
```

But that looks really strange as you wouldn't expect a method like `organisation_has_services_and_information_content?` to be setting instance variables.

You could make `collection_groups` made available using a `helper_method` and then remove the use of the instance variable from the template. Or you could switch it to keep the instance variable in the action method using something like this

```ruby
@collection_groups = build_services_and_information_collection_group
if @collection_groups.blank?
  render status: 404
end
```