PostgreSQL: Fix magic-value code for AbuseFilter
When importing filters, the code is using the magic value
of 'new' for the af_id field, which is of type bigint, to
indicate a not-yet-inserted record.
In MySQL, comparing a bigint to 'new' is legal but always false,
while in PostgreSQL it is illegal, and led to errors when checking
if the filter is hidden.
Fix this by adding special-case code to filterHidden so that it
always returns false for 'new' records. In PostgreSQL this fixes
the error, and in MySQL it avoids a pointless trip to the database.
Should be back patched to 1.23 and 1.24