Add seats_in_use and seats_owed to gitlab_subscriptions

What does this MR do?

Add seats_in_use and seats_owed to gitlab_subscriptions table for data warehouse analysis.

Later MRs would store calculated values in these columns.

Part of customers-gitlab-com#1641 (closed)

Migration

== 20200827085101 AddSeatsCurrentlyInUseInGitlabSubscriptions: migrating ======
-- add_column(:gitlab_subscriptions, :seats_in_use, :integer, {:default=>0, :null=>false})
   -> 0.0040s
-- add_column(:gitlab_subscriptions, :seats_owed, :integer, {:default=>0, :null=>false})
   -> 0.0017s
== 20200827085101 AddSeatsCurrentlyInUseInGitlabSubscriptions: migrated (0.0227s)

== 20200827085101 AddSeatsCurrentlyInUseInGitlabSubscriptions: reverting ======
-- remove_column(:gitlab_subscriptions, :seats_in_use)
   -> 0.0033s
-- remove_column(:gitlab_subscriptions, :seats_owed)
   -> 0.0008s
== 20200827085101 AddSeatsCurrentlyInUseInGitlabSubscriptions: reverted (0.0180s)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done