Add filters to the services page

Add select boxes to the services page to allow filtering of services by department and agency as well as text. Works nicely client-side with JavaScript and GETs a form element when JavaScript isn't working.

I'm not totally happy with the duplication of `toLowerCase().replace(/ /g, '-')` in `dashboards.js` and `/tools/generate-services-list.js` but I can't immediately think of a nicer way of doing it.

I haven't implemented "refine the agencies select box when the department select box is changed", because we don't currently have a way of grouping organisations together (I think we *could* do this, but it would be painful).
I'd consider doing something to shorten this line, either separating it out into a function, or just adding newlines. It was probably already too unwieldy when I wrote it, but it's even worse now.