Warn when po_token, visitor_data and/or inv-sig-helper is not configured (#5202)

* Warn when required configs for playback is missing

* Add link to documentation in warnings

* Direct users to /installation instead

src/invidious.cr

@@ -167,9 +167,16 @@ DECRYPT_FUNCTION =
   if sig_helper_address = CONFIG.signature_server.presence
     IV::DecryptFunction.new(sig_helper_address)
   else
+    LOGGER.warn("WARNING: inv-sig-helper is required for video playback. For more information see https://docs.invidious.io/installation")
     nil
   end
 
+{% for field in %w(po_token visitor_data) %}
+  if !CONFIG.{{field.id}}
+    LOGGER.warn("WARNING: {{field.id}} is required to view and playback videos. For more information see https://docs.invidious.io/installation")
+  end
+{% end %}
+
 # Start jobs
 
 if CONFIG.channel_threads > 0