Add upcoming_reconciliations table and model

What does this MR do?

Create the upcoming_reconciliations table and model. This table will be used to display alerts to customers 7 days before an upcoming reconciliation.

Reconciliations is an upcoming feature where paid subscriptions will be reconciled quarterly. Reconciliation is basically a process where customers are charged for extra users over their subscription quantity.

This table will hold the date when the next reconciliation is to occur, and a date when an alert should start being displayed to the customer.

• For self-managed GitLab instances, the alert will be displayed to admins (https://gitlab.com/gitlab-org/gitlab/-/issues/332041/).

  For SM, there will be only one row, since an SM GitLab instance pays for one subscription. The namespace_id column will be null for SM.

• For gitlab.com (SaaS), customers pay for top-level namespaces, so the alert will be displayed to group owners. (https://gitlab.com/gitlab-org/gitlab/-/issues/215187)

  The namespace_id column in the table is so that we can associate reconciliation dates with namespaces on gitlab.com. We expect about 9,000 - 10,000 rows currently on SaaS.

No history will be kept for SaaS or SM. Once a quarter's reconciliation date passes, the same row will be updated to hold the next quarter's reconciliation date.

Migration output:

== 20210602122213 AddUpcomingReconciliations: migrating =======================
-- create_table(:upcoming_reconciliations)
   -> 0.0065s
== 20210602122213 AddUpcomingReconciliations: migrated (0.0103s) ==============

Revert output:

== 20210602122213 AddUpcomingReconciliations: reverting =======================
-- drop_table(:upcoming_reconciliations)
   -> 0.0019s
== 20210602122213 AddUpcomingReconciliations: reverted (0.0068s) ==============

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Does this MR meet the acceptance criteria?

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Related to #332450