Fix process_subteam_created after adding is_member to SlackSubteam
Fix process_subteam_created after adding is_member to SlackSubteam

This was broken in commit 6f9339b.

I haven't verified that is_member is set to the correct value when a
user group is created, because I don't have access to a slack team where
I can create user groups right know.

rawdigits/wee-slack,trygveaa/wee-slack,wee-slack/wee-slack