Better stats for accreditation status for teams

[punchagan]

• [ ] Extract all the stats computation to one place
• [x] Ignore admins alone, and make sure the calculation matches with what UPAI wants here
• [x] Make it easy to find teams not meeting the minimum requirements.
  • At least 50% of your rostered team (applicable to players and coaches) should hold valid WFDF Advanced Accreditation.
  • The rest of the team should have valid WFDF Basic Accreditation.