When sending an constrct a copy of the PushMessageContent protobuf, add
a SyncMessageContext, and send it to ourselves. Do this for all kinds of
group messages, and individual text/media messages, but not closeSession
messages as the latter are device-specific.
Do not sync messages if we are the primary device, which should only be
supported in development. Normal web clients must be paired with a
android or ios master device, and even in dev, a primary/standalone web
client does not support linking additional devices.
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