Fixed Chrome AJAX submit; switched GET->POST

Together with changes via UI, fixed issue where Chrome only was
refreshing the entire page on highlight-click instead of just updating
the sidebar block (see
https://basecamp.com/2731370/projects/6892159/todos/139848154).
Switched from GET to POST for various reasons (see
https://basecamp.com/2731370/projects/6892159/todos/139718743).