Add additional license information

What does this MR do?

Add additional license information

Provide more information to self-managed users to limit the confusion they might have while renewing. This new information will be referenced in emails coming from the Customer app.

• Add the License#current_active_user_count to the admin dashboard
• add a current_active_user_count helper method

part of: https://gitlab.com/gitlab-org/growth/engineering/issues/40/

Screenshots

Before:

After:

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

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