add email expiration notifications (#365)

config/example-config.yaml

docker/example-docker-config.yaml

go.mod

go.sum

internal/db/cluster/cluster.go

internal/db/dbmigrate/scripts/v0.10.0.pre.sql

internal/db/dbrepo/actionrepo/actions.go

internal/db/dbrepo/mytokenrepo/mytokenrepohelper/helpers.go

internal/db/dbrepo/mytokenrepo/shorttokenrepo/proxytoken.go

internal/db/dbrepo/mytokenrepo/tree/tree.go

internal/db/dbrepo/userrepo/users.go

internal/db/notificationsrepo/notifications.go

internal/db/notificationsrepo/schedule.go

internal/endpoints/actions/actions.go

internal/endpoints/actions/pkg/actions.go

internal/endpoints/configuration/configurationEndpoint.go

internal/endpoints/federation/entityStatement.go

internal/endpoints/guestmode/guestmode.go

internal/endpoints/notification/calendar/calendar.go

internal/endpoints/notification/notifications.go