Improve the migration tool that converts old non-multisite signups to post-2.0 system

This refactoring involves a number of improvements:

- Old signups are identified as those users with user_status=2, instead of
  those users with an 'activation_key' usermeta value. Use of the latter
  technique caused problems with upgrades when plugins had interfered in the
  normal BuddyPress registration flow in such a way as to retain the activation
  key on activated accounts. user_status=2 is a more reliable technique.
- Querying by user_status=2 means that the get_users() query does not have to
  include all usermeta, which makes for a more performant query on large sites.
- The migration routine has been broken out into a separate function
  bp_members_migrate_signups(), and unit tests have been written for it.
- Activation keys are now regenerated for migrated users who did not have an
  existing key in usermeta for some reason.

Fixes #5553

Props imath

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8315 cdf35c40-ae34-48e0-9cc9-0c9da1808c22