Add MINIMAL_ACCESS user statistics

Summary

As discussed in #342125 (closed), we currently have no visibility into the billing of MINIMAL_ACCESS members in the User Statistics

Although we include these members in our billable member counts (unless on Ultimate), they're not added to the statistics, e.g.

So, given the following scenario:

Group A has 4 billable members:

• 1 No project or group
• 1 Minimal Access
• 1 Guest
• 1 Maintainer

Visiting their user stats, they'll see a billable total of 4, but only see a breakdown for 3.

Proposal

As confirmed here #342125 (comment 694781105), we should add this user level to the breakdown to add clarity and visibility for billable members.

1. Add a new column to the user_statistics table for with_highest_role_minimal_access
2. Update the UserStatistics model to include this new level
3. Update the UI to display the count

Considerations

The MINIMAL_ACCESS role is only available in EE (Premium+) so we should consider conditionally displaying the statistics etc. based on that