Current FedSocket implementation has a bunch of problems. It doesn't
have proper error handling (in case of an error the server just doesn't
respond until the connection is closed, while the client doesn't match
any error messages and just assumes there has been an error after 15s)
and the code is full of bad descisions (see: fetch registry which uses
uuids for no reason and waits for a response by recursively querying a
 ets table until the value changes, or double JSON encoding).

Sometime ago I almost completed rewriting fedsockets from scrach to
adress these issues. However, while doing so, I realized that fedsockets
 are just too overkill for what they were trying to accomplish, which is
 reduce the overhead of federation by not signing every message.
This could be done without reimplementing failure states and endpoint
 logic we already have with HTTP by, for example, using TLS cert auth,
or switching to a more performant signature algorithm. I opened
#2262 for further
discussion on alternatives to fedsockets.

From discussions I had with other Pleroma developers it seems like they
 would approve the descision to remove them as well,
therefore I am submitting this patch.

[#1668] App metrics endpoint (Prometheus) access restrictions

Closes #1668

See merge request !3093

Elixir 1.11 compatibility / Phoenix 1.5+

See merge request !3059

[#1668] Restricted access to app metrics endpoint by default. Added ability to configure IP whitelist for this endpoint.
Added tests and documentation.

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

Making this configurable is misleading.

There's really no sound reason to turn this off anyway.

Improvements and fixes for http requests

See merge request !2904

Rich media improvements

See merge request !2944

Fix/2087 metadata

See merge request pleroma/secteam/pleroma!11

not needed options deletion

[#2497] Adjusted media proxy preview invalidation. Allowed client-side caching for media preview. Adjusted prewarmer to fetch only proxiable URIs.
Removed :preview pool in favor of existing :media one. Misc. refactoring.

Use a separate pool for preview requests

- fix for gun worker termination in some circumstances
- pool for http clients (ex_aws, tzdata)
- default pool timeouts for gun
- gun retries on gun_down messages
- s3 upload timeout if streaming enabled