Allow users to link social accounts

[danielcompton]

Jcenter has a mechanism for publicly verifying and linking social accounts and emails to their profile, so that you can see that the danielcompton on Clojars is the same one as the danielcompton on GitHub. I'm not sure if we would want to do this directly or via Keybase? Not sure how closely we want to tie ourselves to Keybase, as a VC startup, they don't have a proven long term track record or clear future.