Sidebar improvements

[frangio]

There's several improvements we can/should make to the sidebar.

• [ ] Remember scrolling position (+ collapsed items?) when navigating between pages.
• [x] Change cursor to pointer when hovering over collapsible items.
• [x] Add indicators for collapsed state.
• [x] Review positioning of sections on shorter viewport heights.
• [x] Fix "current" indicator for items longer than 1 line.

[nventuro]

Our pages are now short enough that we don't get scrollbars anymore (except on the Contracts site). I do think preserving collapsed elements is helpful though.