Polish Tooltip and Menu interactions

+ Make the wrapper button element optional in the tooltip script; only
add it if the target element is not natively focusable
+ Smarter aria for the tooltips: role="tooltip" and proper aria-hidden
states.
+ Much less code to handle additional events for tooltips
+ Fix an issue with Bartender / Prospectus where I mixed up aria-owns
usage
+ Fix an issue in _slats template where metadata/hidden links were
still focusable (should be .hidden, not .sr-only)
+ Site build including the above

Closes #12