Enable use of auditing with non-integer primary keys

Created by: rjw57

The audit models assume that the primary keys of the models being audited will be integers. This is not in general true. Move to storing the string representation of the primary key instead of the integer value.

Add a migration which alters the table. Postgres should be smart enough to coerce the values.

Required to allow auditing the IAR as described in uisautomation/iar-backend#36.