new feature: topic request

There is a new feature on Walltype: you can now specify the postcard topic you want to receive after you send a postcard to another user.

This feature is a big improvement to the workflow on the site. Users get to say exactly what kind of postcard they want to receive, and you are far more likely to receive one on the topic you want when it's your turn to make a request.

Feedback, as always, is welcome.