Add a trial status sidebar widget behind feature flag

What does this MR do?

Adds a new experiment and corresponding feature flag that, when enabled, will show owners of a group with an active trial a new sidebar widget. The new sidebar widget keeps them informed of how many days are left in their trial. When a user interacts with the widget, by clicking/tapping on it, it takes them to the billing/plan-selection page with the hope that users will more frequently & more expediently convert to paying customers.

No documentation or changelog is necessary as this is part of an experiment and is controlled by a feature flag that is disabled until the experiment is ready to be rolled out.

Screenshots

Before (or with FF off)	After (with FF on)

Testing Locally

To test this locally, you’ll need to turn on the check_namespace_plan application setting:

# in the Rails console
> ApplicationSetting.first.update(check_namespace_plan: true)

You may also need to override what ::Gitlab.com? returns to force it to always return true.

After that, you just need to have a group that is currently in a trial. Navigate to the group to see the new trial status widget in the sidebar.

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 #288014 (closed)