mod_rest/openapi: Improve organization and navigation Properties roughly sorted by documentation first. GET paths sorted by complexity. Spacing to help with navigation. Some XML stuff that should arguably be a separate commit, but whatever. Tags.

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.