forbid register with handles='index' and datamask='all'

we cannot allow @register(handles='index', datamask='all' because in this case all data will be masked but never unmasked again. this is certainly seldom what the user want.

changed milestone to %2.0

added BUG category: register/core labels

assigned to @palmb

wait for outcome of #247 (closed)

mentioned in merge request !347 (merged)

mentioned in commit ff8f856a

closed with merge request !347 (merged)