assignments, now topics!

After talking with a few early users of the site, it seems the notion of assignments could be confusing. I've taken several steps to work around this confusion.

This is to say that the project has been given a refined mandate - from the updated front page:

Walltype is a place where you can share postcards with others via postal mail.

This project allows you to send postcards on a variety of topics and, in turn, receive postcards in the mail from other users.

Okay!