Stick public conversation in a page. Fixes #1601

**For discussion only**

This is an attempt at improving the onboarding process, specifically by making it easier to understand what exercism is about before making people log in/sign up, as well as setting the tone for the type of conversation that we're encouraging.

The conversation is real—DataWraith and I actually had these back-and-forths on his real code a few weeks ago.

To try it out, check out the `public-conversation` branch and go to [/conversations/1](http://localhost:4567/conversations/1).

/cc @breethomas @courtney @bunnymatic @michaelavila

(Side note: I was going to try to make it to the Santa Monica Carbon Five Hacknight last night, but work ended up expanding into the evening hours. Any chance any of you Carbon Fivers go to that?)