[MM-51236] - Add opt-in checkbox in self-hosted account creation page (#22709)

* [MM-51236] - Add opt-in checkbox in self-hosted account creation page

* add server impl

* fix lint and translations

* add tests

* improvents

* fix model

* fix mocks

* feedback impl

* update text color

* handle error

* fix tests

server/i18n/en.json

@@ -2591,6 +2591,10 @@
     "id": "api.server.cws.needs_enterprise_edition",
     "translation": "Service only available in Mattermost Enterprise edition"
   },
+  {
+    "id": "api.server.cws.subscribe_to_newsletter.app_error",
+    "translation": "CWS Server failed to subscribe to newsletter."
+  },
   {
     "id": "api.server.hosted_signup_unavailable.error",
     "translation": "Portal unavailable for self-hosted signup."