Unregister Merchant in Merchant Tab

[CarstenWalther]

Currently merchants can be unregistered in the deployment tab.
The merchant token must be provided to unregister the merchant.
In case the merchant token is lost, there is an unremoveable dead merchant entry in the UI.

Unregistering would be easier if we instead add an unregister button to each merchant (in the config>merchant tab). This would be more consistent to unregistering a consumer.