cdr.link add new whatsapp channel

CDR/Link staff just created a WhatsApp support channel for us: https://wa.me/447421000612. Let's include it on the relevant user support pages.

Checklist:

• Tor Browser User Manual
• Support Portal
• Onion Launchpad (thanks, @raya)
• RT template for Russian users (thanks, @nina)
• RT template for Iran users
• Forum post "Iran: Circumventing Censorship with Tor"
• Forum post "Tor blocked in Russia: how to circumvent censorship"