Signed-off-by: marcin mikołajczak <git@mkljczk.pl>

Use Pleroma.Web.Endpoint.url/0 directly instead. Reduces compiler cycles.

Extracted BaseMigrator and BaseMigratorState.

for MediaProxyWarmingPolicy in ConcurrentLimiter

Pleroma.Web.RichMedia.Helpers & Pleroma.Web.MediaProxy

grep -rl '# Copyright © .* Pleroma' * | xargs sed -i 's;Copyright © .* Pleroma .*;Copyright © 2017-2021 Pleroma Authors <https://pleroma.social/>;'

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.

- fetching activity data
- attachment prefetching
- using limiter to prevent overload

Elixir 1.11 compatibility / Phoenix 1.5+

See merge request !3059

Adjusted MediaProxyControllerTest to gracefully fail on missing dependencies. Installation docs update.
Added ffmpeg/imagemagick checks to launch checks (if media preview proxy is enabled). Added documentation on installing optional media / graphics packages (imagemagick, ffmpeg, exiftool).

see changed files for rationale

Fixes race conditions in limit enforcement by putting worker processes
in a DynamicSupervisor