Rename seat calculation methods

What does this MR do?

Prepend calculate_ to method names of seats_in_use and seats_owed (these two methods return dynamically calculated results), because we want to store the calculated value in the database as columns seats_in_use and seats_owed, so we have to reserve methods names for attribute access.

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

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.