Generalize "Congrats!" for communities

As an incentive for people to join communities, I want to reward the
first ten viable communities on Gittip with a "Congratulations!"
message. This brute-forces the calculation of the list of the first ten
communities on Gittip for the purpose of displaying a congratulatory
message to same.
Bump version to 8.2.21
Bump version to 8.2.21-dev