[#2046] Defaulted pleroma/restrict_unauthenticated basing on instance privacy setting (i.e. restrict on private instances only by default).

We don't need it, and local auth fallback has been removed.

* I added the option in config/config.exs
* created a new module lib/pleroma/user/welcome_chat_message.ex
* Added it to the registration flow
* added to the cheatsheet
* added to the config/description.ex
* added to the Changelog.md

This is to model the ambiguous state of most users.

This is used exclusively by MastoFE/GlitchFE now

Mastodon API: ensure the notification endpoint doesn't return less than the requested amount of records unless it's the last page