* 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

Gun is still acting up.

This reverts merge request !2784

This reverts merge request !2677

Closes #1945

The new pooling code just removes the connection when it's down,
there is no need to reconnect a connection that is just sitting idle,
better just open a new one next time it's needed

`:retry_timeout` and `:retry` got removed because reconnecting on failure is
something the new pool intentionally doesn't do.

`:max_overflow` had to go in favor of `:max_waiting`, I didn't reuse the key because
the settings are very different in their behaviour.

`:checkin_timeout` got removed in favor of `:connection_acquisition_wait`,
I didn't reuse the key because the settings are somewhat different.

I didn't do any migrations/deprecation warnings/changelog entries because
these settings were never in stable.

This patch refactors gun pooling to use Elixir process registry and
simplifies adapter option insertion.

Having the pool use process registry instead of a GenServer has a number of advantages:
- Simpler code: the initial implementation adds about half the lines of code it deletes
- Concurrency: unlike a GenServer, ETS-based registry can handle multiple checkout/checkin
requests at the same time
- Precise and easy idle connection clousure: current proposal for closing idle connections in
the GenServer-based pool needs to filter through all connections once a minute and compare their
last active time with closing time. With Elixir process registry this can be done
by just using `Process.send_after`/`Process.cancel_timer` in the worker process.
- Lower memory footprint: In my tests `gun-memory-leak` branch uses about 290mb on peak load (250 connections)
and 235mb on idle (5-10 connections). Registry-based pool uses 210mb on idle and 240mb on peak load

added deprecation warning about using bare domains