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

coderd/database/models.go

@@ -1823,6 +1823,7 @@ type Organization struct {
 	Description string    `db:"description" json:"description"`
 	CreatedAt   time.Time `db:"created_at" json:"created_at"`
 	UpdatedAt   time.Time `db:"updated_at" json:"updated_at"`
+	IsDefault   bool      `db:"is_default" json:"is_default"`
 }
 
 type OrganizationMember struct {