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 @hongminhee in
[IRC](https://botbot.me/freenode/gittip/msg/9818883/), we're dropping
the "Congratulations!" text entirely.
Merge pull request #1907 from gittip/take-out-congrats

Remove "Congrats!" text for first N communities