Please automatically post the `Rendered` link

[joshtriplett]

People regularly post RFCs without a Rendered link pointing to the rendered Markdown, or post with a link that goes to a specific commit hash rather than the branch. Please consider putting this functionality in rfcbot itself.

[Centril]

So the plan is for @rfcbot to make a first post whenever someone makes an RFC;
The rendered link and FCP boxes will be included in that first post so that you don't have to scroll for it. :)
This will also help with #198 eventually.