FeeSettings logic contract could brick tokens

[malteish]

Problem

During creation and fee settings changes, a token contract checks if the new fee settings contract implements the interfaces required. A fee settings logic contract will pass this test, but is controlled by 0x. This means a token could never migrate away from this contract again.

Mitigation

avoid accidentally proposing a fee settings logic contract to a token, e.g. by reading the fee settings owner in the web app and rejecting if it is 0x (or not the owner we expect).

Fix

Have the token enforce that the owner of the fee settings contract is not 0x.

[CJentzsch]

The fix is more of a mitigation, as the non-0x address could be an unused address as well, but I think the fix is good enough. We should put it in.