feat(coderd/rbac): make organization-member a per-org system custom role (#21359)

Migrated the built-in organization-member role to DB storage so it can be customized per org.

Closes https://github.com/coder/internal/issues/1073 (part 1)

coderd/members.go

@@ -393,9 +393,10 @@ func convertOrganizationMembers(ctx context.Context, db database.Store, mems []d
 	}
 
 	customRoles, err := db.CustomRoles(ctx, database.CustomRolesParams{
-		LookupRoles:     roleLookup,
-		ExcludeOrgRoles: false,
-		OrganizationID:  uuid.Nil,
+		LookupRoles:        roleLookup,
+		ExcludeOrgRoles:    false,
+		OrganizationID:     uuid.Nil,
+		IncludeSystemRoles: false,
 	})
 	if err != nil {
 		// We are missing the display names, but that is not absolutely required. So just