Support toggling "Invisible mode"

[chrisbobbe]

This setting: https://zulip.com/help/status-and-availability#invisible-mode

Implemented in zulip-mobile in zulip/zulip-mobile#5531

Related issues

• Presence data, and report presence #196

[chrisbobbe]

When this is done we should update the Help Center doc. We removed instructions for the legacy app when preparing to launch the new app: zulip/zulip@815a522

[chrisbobbe]

Someone asked for this feature in the first week of the launch period. (A Play Store review)

[timabbott]

This was requested via an issue in the legacy repository here: zulip/zulip-mobile#5927.

[chrisbobbe]

Do we have a design for this? @terpimost @alya

Started a thread: #mobile-design > presence: invisible mode @ 💬