[Fix bug 1059481] Add required dependency in autovouch datamigration.

* Data migration #0070 in users app depends on groups_groupmembership table.
Merge pull request #1078 from johngian/1059481

[Fix bug 1059481] Add required dependency in autovouch datamigration.
[Fix bug 1053046] Add management command to vouch former staff.
Merge pull request #1076 from johngian/vouch_former_staff

[Fix bug 1053046] Add management command to vouch former staff.