feat: implement 'is_default' org field (#12142)

The first organization created is now marked as "default". This is
to allow "single org" behavior as we move to a multi org codebase.

It is intentional that the user cannot change the default org at this
stage. Only 1 default org can exist, and it is always the first org.

Closes: https://github.com/coder/coder/issues/11961

codersdk/organizations.go

@@ -30,6 +30,7 @@ type Organization struct {
 	Name      string    `json:"name" validate:"required"`
 	CreatedAt time.Time `json:"created_at" validate:"required" format:"date-time"`
 	UpdatedAt time.Time `json:"updated_at" validate:"required" format:"date-time"`
+	IsDefault bool      `json:"is_default" validate:"required"`
 }
 
 type OrganizationMember struct {