Use WhatsApp user-agent for rich media by default

Related https://gitlab.com/soapbox-pub/rebased/-/issues/180

CHANGELOG_soapbox.md

@@ -6,6 +6,11 @@ For changes to Pleroma, see `CHANGELOG.md`
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## Unreleased
+
+### Fixed
+- Link previews not working for some websites. Let rich media user-agent be configurable (with `:pleroma, :rich_media, user_agent: "whatever"`).
+
 ## [3.0.0] - 2022-05-24
 
 Based on Pleroma 2.4 develop.

config/soapbox.exs

@@ -53,3 +53,6 @@
 
 # Background migration performance
 config :pleroma, :delete_context_objects, sleep_interval_ms: 3_000
+
+# Pretend to be WhatsApp because some sites don't return link previews otherwise
+config :pleroma, :rich_media, user_agent: "WhatsApp/2"

lib/pleroma/web/rich_media/helpers.ex

@@ -16,7 +16,18 @@ defmodule Pleroma.Web.RichMedia.Helpers do
     recv_timeout: 2_000
   ]
 
-  defp headers, do: [{"user-agent", Pleroma.Application.user_agent() <> "; Bot"}]
+  defp headers do
+    user_agent =
+      case Config.get([:rich_media, :user_agent], :default) do
+        :default ->
+          Pleroma.Application.user_agent() <> "; Bot"
+
+        custom ->
+          custom
+      end
+
+    [{"user-agent", user_agent}]
+  end
 
   @spec validate_page_url(URI.t() | binary()) :: :ok | :error
   defp validate_page_url(page_url) when is_binary(page_url) do