Revert removal of "unecessary" assignments to `$_POST['id']`" (issue #762)

Turns out it was needed by the 'unique[]' validation rule,
which I now understand (regardless of whether I approve of it).

TODO: I still think the one in the roles controller is going
to cause breakage on certain databases, where inserting 0 into
an AUTO_INCREMENT/equivalent will simply insert a 0 value.

Completely reverts 5f9fff85bc2f191c0b911bf317a50dfa6e18ccaa.
Reverts non-core part of 0306fe54afc705059407c58014c82d6aa034c54.