Optionally hide online status

[MomentQYC]

Description:

For privacy reasons, many people do not like to expose their online status.

[DMRobertson]

To disable presence at a homeserver level, set

presence:
    enabled: false

(see config manual).

For finer-grained controls at the server level, see #8055.

Additionally, the spec's wording suggests that clients can opt out of sending presence:

User’s presence state is represented by the presence key, which is an enum of one of the following:

offline: The user is not connected to an event stream or is explicitly suppressing their profile information from being sent.

I have no idea if Synapse correctly implements that, though.

[deepbluev7]

Synapse currently doesn't allow you to set yourself to offline. I have some patches in progress to fix that and a few other presence bits though.