Daily background job to run the Seat Link service

Problem

As described in https://gitlab.com/gitlab-org/customers-gitlab-com/issues/1139, self-managed instances will need to routinely send seat link information to the Customers Portal. The instance needs to be configured call the "Seat Link" data service on a daily basis.

Proposal

Create a sidekiq worker responsible for calling the new "Seat Link" service. Schedule this worker to be run on a daily basis (at midnight?) using cron, probably similar to the GitlabUsagePingWorker setup.

Result

A scheduled job to run the "Seat Link" service on a daily basis.