translation: Translate new user messages to realm's default language.
translation: Translate new user messages to realm's default language.

Previously, automated stream messages for new user signups were not
being translated into the realm's default language for said messages.

Moves `override_language` context manager so that it wraps the
new user message content in `notify_new_user` and topic string in
`send_message_to_signup_notification_stream`.

Fixes #22510.

rht/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip