Improve /department/id search view for mobile

[abandoned-prototype]

The view in mobile currently shows only the filter and one has to scroll down to see any results:

Additionally the results are paginated and after loading each new page the user has to scroll down again to see the results:

Two improvements I have in mind:

1. Make the whole Filter officers collapsible and default to collapsed if the user is on mobile
2. Implement continuous scrolling instead of the current pagination - this will require a new API I imagine. On desktop, I imagine if we can make it work with the filter on the left staying fixed and only the content on the right scrolling down, I think we should have the continuous scrolling for all devices.

[michplunkett]

This PR does not cover all of the things asked for, but it does address some small pain points for mobile: #965