TURN needs to be easier to set up (SYN-76)

[matrixbot]

Whilst we support TURN now, it's dauntingly complicated for a casual homeserver user to set up.

We need to either make it easier to use existing external TURN services and/or host our own public TURN service (as Matrix.org) for the betterment of humanity.

(Imported from https://matrix.org/jira/browse/SYN-76)

(Reported by @ara4n)

[matrixbot]

Jira watchers: @ara4n

[richvdh]

related: #1519.

[clokep]

We added some documentation for this since this issue was filed: https://matrix-org.github.io/synapse/latest/turn-howto.html

I'm going to close this lacking specific details of what needs to be improved.