mod_http_oauth2: Tweak method of centering the UI The percentage here was relative to the viewport width, which on some very wide screens may put the UI slightly outside of the view, requiring scrolling to see. By using a unit relative to the height of the viewport, this is avoided and should work better. But no guarantees, it's still possible to resize the browser or adjust font sizes so that the UI goes out of view.

This module writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.