Chain policies

- The `:pleroma, :instance, :rewrite_policy` can now be either a policy
or a list of policies
- Made a behaviour for MRF policies

lib/pleroma/web/activity_pub/mrf.ex

@@ -0,0 +1,21 @@
+defmodule Pleroma.Web.ActivityPub.MRF do
+
+  @callback filter(Map.t) :: {:ok | :reject, Map.t}
+
+  def filter(object) do
+    get_policies()
+    |> Enum.reduce({:ok, object}, fn
+      (policy, {:ok, object}) ->
+        policy.filter(object)
+      (_, error) -> error
+    end)
+  end
+
+  def get_policies() do
+    Application.get_env(:pleroma, :instance, [])
+    |> Keyword.get(:rewrite_policy, [])
+    |> get_policies()
+  end
+  def get_policies(policy) when is_atom(policy), do: [policy]
+  def get_policies(policies) when is_list(policies), do: policies
+end