close websocket connections on ping/pong timeout

we recently added ws pings for keep alive,
and we can now use the longs to close dead connections.

This can apparently happen when connections are not closed cleanly,
e.g. on VPNs.

Setting ws_ping_timeout = 0 disables the ping mechanism altogether.
Merge pull request #6406 from minrk/celltoolbar-rendered

don't hide cell toolbar on rendered text cells
Merge pull request #6302 from minrk/ws-heartbeat-timeout

close websocket connections on ping/pong timeout
make the default url customizable

this is the page redirected to from `/`,
which defaults to `/tree`.

also changes the redirect code to 302 from 301,
which should cause less grumbling about caching.
remove unused TreeRedirectHandler
Merge pull request #6413 from minrk/default-uri

make the default url customizable
Move md-cell display logic to css