[#1666] Related requests typeahead search

Fixes #1666.

> Make the typeahead search panel closable

~~I'm not sure if we have an existing UI for doing this. Open to suggestion.~~

Found that we have the close button on the "link-to-this" modal on the request page.

> I guess it would be really good if the typeahead panels had some animation to explain where they are coming from, a slide-down/slide-up would be quite decent

Can we do this with CSS? Now the results are limited to 3 it doesn't seem quite as jarring to have them just appear, so I don't think we need to be majorly compatible with old browsers here.