This fixes two bugs:
* If a user is not subscribed to a default stream, he or she would not
be have the option to invite users to that default stream.
* The initial streams checked in the invite modal were the
non-invite-only streams the user was subscribed to, not their
default streams.
Fixes: #4209.
In 06615bee00, we accidentally
introduced a duplicate HTML ID for #stream-checkboxes, which in turn
caused the "invite users" page to no longer work. This fixes that
problem.