Let people check feature overlap

[DanielOaks]

For instance, for all the servers that support the sasl cap, which other capabilities do they support?

Interesting to look at, but it probably requires digging into the stats at runtime rather than presenting a massive blob of data to the page, which requires us exporting a sanitized data set for js to eval. I dunno, will look at how useful this is and how much work it would be to implement.