The push preview route now reads the device policy of the login it
serves (its scopes worked out from the login's own JMAP session, every
non-personal account counted as a group) and under
`pushWithoutContent` asks the mail server for the message's ids only,
so sender, subject and text never reach the notification. The service
worker shows "New mail" for such an answer. A policy that can't be read
counts as on.
The three existing push preview route tests now mock the config manager
with the default policy: without it the route can't read a policy in the
test environment and holds the text back, which those tests didn't mean
to cover. No assertion changed.
Linus Rath
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