template: Tenancy

[ascott18]

### Tasks
- [x] Switching: Choose tenant screen
- [x] Invitations: Generate link
- [x] Invitations: landing page for invitation link. Use existing account (MVP), or sign in with external (MVP), or create local account (future)
- [x] Self-service: tenant creation
- [x] Switching: link to switching page from top-right menu
- [ ] Hostname: middleware to set tenantID from host name
- [ ] Hostname: tables for hostnames
- [ ] Hostname: send correct hostname in invitation links
- [x] Global Admin: All-tenant admin area
- [x] Global admin: Create tenants
- [ ] ~~Global admin: Add users to tenants~~
- [x] Handle audit logs

[ascott18]

Use Cases

• Tenant decided per domain. No tenant switching, no determination based on user source (AAD/Google). Used for sites that have a public, unauthenticated aspect to them so that public users can be routed to a specific tenant.
  • ❤Users are explicitly invited to tenant (Goat)
  • 💜OR users are permitted to freely sign up to that tenant, or link their existing account to that count
• Single domain. Can switch between tenants with a dropdown, maybe prompted on sign-in. Options:
  • 💙Tenant strictly tied to external sign in, tenants either created automatically based on external provider or explicitly created by "super admin" or a sign-up form. (Myriad, Wiki, STC)
  • 💛Tenant membership granted by being invited by an existing admin within that tenant, or by the "super admin" (AIS, CWA)
  • 💚Hybrid: 💛 + Automatic membership to home tenant. (Myriad desired)

Options (each actual template flag marked with ⭐, not all combinations are valid)

• ⭐Base
  • Tenant table
  • TenantMembership table (User-Tenant many-to-many)
  • AppUser { bool IsGlobalAdmin }
  • Global query filter & save interceptor
  • TenantBase class
• Membership:
  • ⭐Invitations from admins within the tenant
  • ⭐Invitations from Global Admin
  • ⭐Self-service sign-up or link existing account
  • ⭐Implicit from AAD/GSuite
• ⭐Tenant decided by:
  • hostname
  • switching/prompt
  • future: path segment
• Tenant creation:
  • ⭐Self-service sign-up form
  • ⭐Automatic from AAD/GSuite
  • ⭐super admin