[#18] Make the sidebar collapsible

This PR makes the Search page sidebar collapsible. The collapsed state is saved in local storage so that it's persisted between reloads.

When the sidebar collapses, all the contained sections are collapsed too, only the button icons remaining visible. Any indicators that were displayed on the sections will now become visible on the buttons, with the correct color.

Clicking on one of the buttons expands both the sidebar and the corresponding section.