We want to remind unverified users to verify their emails:
once after 24 hours has passed and again after 5 days has passed.
Add mailservice.Service to satellite core because it is needed by the
chore for sending emails. To add the mailservice.Service to the core,
we create a helper function in satellite/peer.go to avoid duplicating
the code in both api.go and core.go. In addition to the chore, this
change adds methods to users.DB to get unverified users in need of
reminder.
Change-Id: I4e515bdf43f922788b4f965b2efb34fa32288bd1
