Added CouponInput component

no-issue

This can be used in the subscribe pages to pass coupon info through

ghost/members-api/static/auth/components/CouponInput.js

@@ -0,0 +1,19 @@
+import FormInput from './FormInput';
+import { IconName } from './icons';
+
+export default ({value, disabled, error, children, onInput, className}) => (
+    <FormInput
+        type="text"
+        name="coupon"
+        label="coupon"
+        value={value}
+        error={error}
+        icon={IconName}
+        placeholder="Coupon..."
+        required={false}
+        disabled={disabled}
+        className={className}
+        onInput={onInput}>
+        {children}
+    </FormInput>
+);