[ENG] Add current trial status for namespace to sidebar

Child of #220930 (closed).

• Create a new experiment feature flag
• Pivot the experiment on the Group (namespace) rather than User
• Only include the new sidebar item for owners of the Group who are not members (of any sort) or guests of any namespace which currently has a paid account.
• No popover on hover, we’ll make the whole sidebar item clickable and take them to the billing page for that namespace
• Track which namespaces are in the control vs. experimental cohorts – will be taken care of by using record_experiment_subject(…) once that becomes available
• [-] Track click rate of the info-button – will be handled by a standard Snowplow event
• [-] Track the trial upgrade rate of the control vs. experimental cohorts – will use the new record_experiment_conversion_event(…) helper method.

Moved the last two items to their own issue: #300728 (closed)