This is a reboot of #217 based on mailing list discussion. It shows either a banner for SotM Birmingham or for SotM US, static, 50/50 split. (A version which faded between the two on a 10-second timer was also considered, but found to be visually distracting.)
Remove commented out donation section
There's an displayed donation section further down in
Add SotM ads
This approach uses Ruby's [sample](http://ruby-doc.org/core-2.0/Array.html#method-i-sample) to pick randomly between the Birmingham and San Francisco ads. 50 % of users will see the Birmingham ad, 50 % of users will see the San Francisco ad.