Re-charter the Authorization IG as a single venue

Fold the auth-wg-* sub-group channels and the Enterprise-Managed
Authorization IG into #auth-ig with one thread per topic, make the
biweekly call agenda-driven (slots requested and handed out each time,
thin agenda cancels), bring SEP/draft/demo feedback explicitly into
scope, and record the two standing questions asked of every problem
pitch before a SEP is drafted.