Add RSS feed autodetection

[bonfield]

1. Go to RSS RSS RSS RSS
2. In the box under "Add your RSS feeds", paste in the URL for any Buttondown-hosted landing page or archive, e.g.: https://buttondown.com/monteiro or https://buttondown.com/monteiro/archive/
3. You will see an error at the right of the screen that starts with "Failed to load this feed"

[jmduke]

We don't actually do RSS feed detection! We expect you to supply e.g. https://buttondown.com/monteiro/rss. That being said... not a bad idea! Keeping this around.

[jmduke]

Closed via f133da8