Merge branch '534_federation_targets_reachability' into 'develop'

[#534] Unreachable federation targets retirement

Closes #534

See merge request pleroma/pleroma!703

diff --cc config/config.exs
Simple merge

diff --cc docs/config.md
Simple merge

diff --cc lib/pleroma/web/activity_pub/activity_pub.ex
Simple merge

diff --cc test/support/http_request_mock.ex
Simple merge

diff --cc test/web/activity_pub/activity_pub_test.exs
Simple merge