Add ability to set po_token and visitordata ID (#4789)

This PR adds two new config option, to pass a PO token (config 'po_token') and
a visitor ID (config 'visitor_data') to Youtube. These two strings are required
to play videos using the WEB client.

Warning: These strings gives much more identifiable information to Google!

If the po_token setting is filled in, then the WEB client is used. If not, the
Android client is used. TvHtml5ScreenEmbed will still be used as a fallback.

Script for generating po_token and visitor_data:
https://github.com/iv-org/youtube-trusted-session-generator

Helps with issue 4734

config/config.example.yml

@@ -186,6 +186,18 @@ https_only: false
 ##
 # use_innertube_for_captions: false
 
+##
+## Send Google session informations. This is useful when Invidious is blocked
+## by the message "This helps protect our community."
+## See https://github.com/iv-org/invidious/issues/4734.
+##
+## Warning: These strings gives much more identifiable information to Google!
+##
+## Accepted values: String
+## Default: <none>
+##
+# po_token: ""
+# visitor_data: ""
 
 # -----------------------------
 #  Logging