Build out Positions CRUD and rework the Roles page

Positions had list/create/edit already stubbed as list-only; give it real
forms, with a check mirroring the management_position_implies_staff_position
constraint so a bad combination reads as a form error, not a 500. Roles now
groups by role (excluding the MEMBER everyone holds automatically, which was
just noise), grants via a modal instead of a separate page, and its member
picker is a small typeahead combobox instead of a long native <select>.

management/context_processors.py

@@ -36,6 +36,8 @@ _NAV_SECTIONS = {
     "membership_mark_fully_paid": "membership_list",
     "membership_record_payment": "membership_list",
     "position_list": "position_list",
+    "position_create": "position_list",
+    "position_update": "position_list",
     "role_list": "role_list",
     "role_create": "role_list",
     "role_revoke": "role_list",