Add deprecation warning mechanism.

lib/pleroma/application.ex

@@ -22,6 +22,8 @@ def user_agent() do
   def start(_type, _args) do
     import Cachex.Spec
 
+    Task.start(&Pleroma.DeprecationWarnings.warn/0)
+
     # Define workers and child supervisors to be supervised
     children =
       [

lib/pleroma/deprecation_warnings.ex

@@ -0,0 +1,20 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2019 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.DeprecationWarnings do
+  require Logger
+
+  def check_frontend_config_mechanism() do
+    if Pleroma.Config.get(:fe) do
+      Logger.warn("""
+      !!!DEPRECATION WARNING!!!
+      You are using the old configuration mechanism for the frontend. Please check config.md.
+      """)
+    end
+  end
+
+  def warn do
+    check_frontend_config_mechanism()
+  end
+end