Add an ARK revisions audit trail

[mjordan]

Although larkm doesn't manage users, other applications may, and this field will let those applications add user IDs to each ARK record. Default value should be 'none'.

[mjordan]

Would be better to track changes to ARKs as well. We can do this by adding a second table to the db and in it store create/update dates, user info, plus the previous version of the ARK's record.