add native beforeUnload handlers

[feederbox826]

partially addresses #594

doesn't work with SPA navigation because

it's "unstable", reactrouter does not intercept it, it does not work on SPAs, the PR (s) are dead, is still unstable

If someone else who is more versed and has more patience for SPAs and React wants to improve, it they are more than welcome to. In the meantime I will just intercept navigate events

[feederbox826]

will rebase