Remove OnboardingRequirement.order; restrict checklist actions to admin/MEMBER_ADMIN

Every active requirement blocks equally and there's no set order to complete
them in, so the configurable "order" field (and its ordering-by-number) is
gone -- requirements list alphabetically now, both in the admin UI and the
Onboarding requirements settings page.

Also closes a real permission gap found while checking this: marking a
checklist item complete, bypassing it, or reopening it (management/views.py's
MemberRequirementCompleteView/BypassView/IncompleteView) was open to *any*
staff member with page access, not just admin/MEMBER_ADMIN, despite the
member detail page's own Documents card implying otherwise. Switched all
three to MemberAdminRequiredMixin and hid the corresponding buttons/dialog
from anyone who can't use them. The Sign-up page's Bypass action was already
admin-only end to end (the whole page is ClubAdminRequiredMixin-gated), so
no change in practice there.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ECGMEwrc2k4D8VQuwjstj9

static/css/app.css

@@ -2853,6 +2853,9 @@
       }
     }
   }
+  .isolate {
+    isolation: isolate;
+  }
   .stack {
     @layer daisyui.l1.l2.l3 {
       display: inline-grid;