Increment the max seats and seats owed values in real time as users are added

What does this MR do?

Instead of waiting for the daily script that updates the max_seats_used counter we're updating it from every place where Group or Project members are created or updated.

Calculating the total number of billable members can be an expensive task for large groups like gitlab-org. That's why we're using a feature flag, we need to disable this feature in case we observe some performance issues.

Closes #199891 (closed)

Screenshots

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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team