Provide more visibility to the Forum on our traditional support channels

I am filing this ticket to account for the various tasks related to making the Forum more visible on our traditional user support channels

• Create article on RT (thanks, @gus!) (I am also exploring the idea of making this a auto-reply on frontdesk)
• Add a link to the forum to tormodbot's welcome message on IRC #tor (and to the channel description as well)
• Add a link to the forum in the description of the #tor:matrix.org channel.