Remove "Congrats!" text for first N communities

Now that we've dropped the threshold for "viable" communities from 150 to 50 (#1316), that throws a wrench in the "Congratulations!" text we had for the first communities to reach the 150 mark on Gittip. After talking with @dahlia in [IRC](https://botbot.me/freenode/gittip/msg/9818883/), we're dropping the "Congratulations!" text entirely.