html: add policy which transforms inline images to pass through the media proxy

lib/pleroma/html.ex

@@ -142,3 +142,34 @@ defmodule Pleroma.HTML.Scrubber.Default do
 
   Meta.strip_everything_not_covered()
 end
+
+defmodule Pleroma.HTML.Transform.MediaProxy do
+  @moduledoc "Transforms inline image URIs to use MediaProxy."
+
+  alias Pleroma.Web.MediaProxy
+
+  def before_scrub(html), do: html
+
+  def scrub_attribute("img", {"src", "http" <> target}) do
+    media_url =
+      ("http" <> target)
+      |> MediaProxy.url()
+
+    {"src", media_url}
+  end
+
+  def scrub_attribute(tag, attribute), do: attribute
+
+  def scrub({"img", attributes, children}) do
+    attributes =
+      attributes
+      |> Enum.map(fn attr -> scrub_attribute("img", attr) end)
+      |> Enum.reject(&is_nil(&1))
+
+    {"img", attributes, children}
+  end
+
+  def scrub({tag, attributes, children}), do: {tag, attributes, children}
+  def scrub({tag, children}), do: children
+  def scrub(text), do: text
+end