Switch newsletter form to SvelteKit Form Actions

- remove api endpoint and use latest SK form actions
- create reusable server page function for form handling
- handling the errors in the server function

src/routes/api/newsletter/+server.ts

@@ -1,30 +0,0 @@
-import { error } from '@sveltejs/kit'
-import type { RequestHandler } from './$types'
-import { NEWSLETTER_API_TOKEN, NEWSLETTER_LIST_ID } from '$env/static/private'
-
-
-export const POST: RequestHandler = async ({ request }) => {
-    try {
-        const body = await request.text()
-
-        if (body) {
-            const req = await fetch(`https://emailoctopus.com/api/1.6/lists/${NEWSLETTER_LIST_ID}/contacts`, {
-                method: 'POST',
-                headers: { 'Content-Type': 'application/json' },
-                body: JSON.stringify({
-                    api_key: NEWSLETTER_API_TOKEN,
-                    email_address: body,
-                })
-            })
-            const res = await req.json()
-
-            if (res) {
-                return new Response(JSON.stringify({
-                    code: res.error ? res.error.code : res.status
-                }))
-            }
-        }
-    } catch (err) {
-        throw error(403, err.message)
-    }
-}