Give turn on 443 another run #213

@alangecker In #73 we decided that bringing back turn with default ports (in #126) was a sensible compromise, since other project had forfeited the nginx based demultiplexing approach. I don't know how reliable the current turn solution (with default ports) works for most people. Some Github issues seem to indicate it needs a rework anyway (#189). I just stumbled upon the sslh repo, which works well with docker and people there have a working Coturn setup on 443 (albeit only for encrypted connections yrutschle/sslh#246). Advantage: A dedicated/proven tool for demultiplexing may be more reliable and easier to maintain. Enabling coturn could be made default (at least with auto-https enabled) resulting a more complete bbb-docker AIO package. AFAICT a quick proof of concept could be plugged together within an hour. However I'm not at all experienced in analyzing and debugging turn connections. Is anyone more capable interested in embarking on this endeavour? Or maybe this can be considered in #207 when fixing #189.