[#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

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