Move simple, useful methods from helper to model

What does this MR do?

Moves a collection of simple & useful methods for calculating information about a GitlabSubscription trial from a helper that was recently added into the GitlabSubscription model itself, and then delegating them into the Group model for ease-of-access. This cleans up the helper code quite a bit (as we kept having to pass the group object around) and allows these methods to be more easily discovered & reused by other developers.

Note: this MR does not need a changelog entry as it is a “developer-facing change.” See: https://docs.gitlab.com/ee/development/changelog.html#what-warrants-a-changelog-entry

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

Related to #296686 (closed), #288014 (closed), & #288015 (closed)