Add department filtering to services page

This commit adds a select box to the services page to filter
services based on their department.

It generates an array of departments using a dashboard collection
which it uses to populate the select box. We create slugs based on
the department's abbreviation.

This works in browsers with and without JavaScript by adding a
submit button to the form.
Pluralise 'service(s)' correctly on services page
Add agency filtering to services page

This is broadly similar to the departmental filtering.
Show current filter in text title on services page

When the user has filtered the list of services, update the text
underneath the count of services to show what they're currently
filtering as.