Since da8f4bc0e back in August, this control flow has caused
`flags.active_mobile_push_notification` to be cleared if we don't send
these `remove` messages at all, and if we send them directly to GCM...
but not if we send them via the Zulip notification bouncer.
As a result, on a server configured to send `remove` notification-messages
via the bouncer, we accumulate "active" messages and never clear them.
If the user then does `mark_all_as_read`, we end up sending a `remove`
for each of those messages again, and all in one giant burst. We've
seen puzzling bursts of hundreds of removals pass through the bouncer
since turning on removals on chat.zulip.org; it's likely many of them
are caused by this bug.
This issue was made more acute with f4478aad5, which unconditionally
enabled removals.
Test added by tabbott.
0213aa0b16