This change is to make a new user to be a member of default groups, when the user is registered or added to a local-site.
Works done are:

1. handle newly registered user

  1) catch user_registered djblets.auth.signals.user_registered signal invoked when a new user is registered

  2) add the new user to default groups

2. handle new LocalSite-User relation

  1) catch m2m_changed (many-to-many relation changed) signal invoked when a LocalSite-User relation changed

  2) invoke local_site_user_added for new relations between LocalSite and User

  3) make a receiver of local_site_user_added, to add this newly added user to the LocalSite's default groups

Browser tests done:

make new local site
make new group, addign local the local site to it, and make it a default group
make new group and addign local the local site to it (do not make it default group)
add a user to the local site
see if the user is added to group created in #2
see if the user is not added to group created in #3

Unit Tests done:

check if _add_default_groups signal handler is called properly when a user is registered
check if _add_default_groups signal handler is called properly when local_site.users.add(user)
check if _add_default_groups signal handler is called properly when user.local_site.add(local_site)
check if _add_default_groups adds global default review_groups when a user is registered
check if _add_default_groups adds a local_site's default groups when local_site.users.add(user) or user.local_site.add(local_site)

Edge cases:

there's no default group
there's no non-default group