At the moment, PMs to groups with default notification level set to
`watching_first_post` do not generate "emailable" notifications. This happens
because, topic user notification level which is indirectly derived
from the group's default notification level is set to `tracking` if the
group's notification level happens to be `watching_first_post`.
This leads to a `group_message_summary` notification being created
instead of a `private_message` notification which results in no email
alerts being sent when a topic is created.
As this `watching_first_post` --> `tracking` switcheroo appears to be
intentional instead being a bug, this change extends `PostAlerter`'s
`notify_pm_users` method to create a `private_message` notification for
first posts created in a `watching_first_post` group even if the topic
user notification level is set to `tracking`
c45eb8a618