This is done by HTTP POSTing to a location specified by
`sv_modhelp_url`. We also provide a `src/modhelp/server.py` which can
use theses POSTs to forward them to Discord servers.
The POST contains a JSON object payload, with the keys `"port"` which
contains the server port, `"player_id"` which contains the calling
player's client ID, `"player_name"` which contains the calling player's
nick and `"message"` which is the user-specified message.
Make JSON-escaping function public, add tests and fix bugs uncovered by
these tests.
Supersedes #1129.
