Use Jason for rendering responses

Although Jason readme says Phoenix 1.4+ already does it by default, [it actually does it only for new projects](3bfb9f6e90/lib/phoenix.ex (L58-L59))

config/config.exs

@@ -373,6 +373,8 @@ config :pleroma, :chat, enabled: true
 
 config :phoenix, :format_encoders, json: Jason
 
+config :phoenix, :json_library, Jason
+
 config :pleroma, :gopher,
   enabled: false,
   ip: {0, 0, 0, 0},

lib/pleroma/healthcheck.ex

@@ -9,6 +9,7 @@ defmodule Pleroma.Healthcheck do
   alias Pleroma.Healthcheck
   alias Pleroma.Repo
 
+  @derive Jason.Encoder
   defstruct pool_size: 0,
             active: 0,
             idle: 0,