Admin/Staff roles rework

[calebyhan]

Now with Onyen integration admins/staff stuff needed reworking

Changes:

• Last-admin guardrail
• Admins can delete other admins
• Tests

[github-actions]

Test Results

627 tests  +9   627 ✅ +9   55s ⏱️ +4s
  1 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 771d4d1. ± Comparison against base commit 064674a.

This pull request removes 3 and adds 12 tests. Note that renamed tests count towards both.

tests.models.test_other_models.TestBudgetDataModel ‑ test_updated_by_not_nullable
tests.models.test_other_models.TestCalendarEventModel ‑ test_calendar_event_requires_created_by
tests.models.test_other_models.TestCalendarEventModel ‑ test_created_by_not_nullable

tests.models.test_other_models.TestBudgetDataModel ‑ test_updated_by_nullable
tests.models.test_other_models.TestCalendarEventModel ‑ test_calendar_event_allows_null_created_by
tests.models.test_other_models.TestCalendarEventModel ‑ test_created_by_nullable
tests.routes.test_admin_accounts.TestDeleteAdminAccountWithAuthoredContent ‑ test_deleting_admin_cascades_section_membership
tests.routes.test_admin_accounts.TestDeleteAdminAccountWithAuthoredContent ‑ test_deleting_admin_nulls_app_config_updater
tests.routes.test_admin_accounts.TestDeleteAdminAccountWithAuthoredContent ‑ test_deleting_admin_nulls_budget_data_updater
tests.routes.test_admin_accounts.TestDeleteAdminAccountWithAuthoredContent ‑ test_deleting_admin_nulls_calendar_event_creator
tests.routes.test_admin_accounts.TestDeleteAdminAccountWithAuthoredContent ‑ test_deleting_admin_nulls_static_page_editor
tests.routes.test_admin_accounts.TestLastAdminProtection ‑ test_admin_can_delete_another_admin
tests.routes.test_admin_accounts.TestLastAdminProtection ‑ test_admin_can_demote_another_admin
…

[MasonMines2006]

LGTM