Broflake: add a social share feature

[noahlevenson]

Social word-of-mouth (and virtue signaling) is critically important for volunteer initiatives. Broflake should have a prominent low-friction mechanism for social sharing.

IMO, it's arguable that it's even more important that users share Broflake than that users run Broflake. I think we should put a big CTA to "share your impact" in the header. In the future, this could generate the "baseball card" image. Today, it could just assemble some text from the user's stats?