Adds feature to permit e.g., local admins and community moderators to automatically follow all newly registered accounts

In practice, it was already removed half a year ago, but the description
 and cheatsheet entries were still there.

The migration intentionally does not use ConfigDB.get_by_params, since
this will break migration code as soon as we add a new field is added
 to ConfigDB.

Closes #2086

Reported by:	trevoke

Also remove the example from the cheatsheet, there is no need for it
when the types are simple

Rich media improvements

See merge request !2944

Fix/2087 metadata

See merge request pleroma/secteam/pleroma!11

not needed options deletion

[#2046] Added test for pleroma/restrict_unauthenticated defaults on private instance. Updated docs and changelog.

* 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 reverts merge request !2825

This reverts merge request !2820