MT#4967 Let subadmins steal numbers

A subadmin can use the field alias_select to select any number of alias
numbers, which he is allowed to manage. If he selects one, it moves
to this subscribers alias_numbers. If he unselects one, it moves to his
own alias numbers.
MT#4967 Disallow subscriberadmin to change status

see 4967#c28533
MT#4967 Do not show password on edit

but update it, if one is supplied
sub edit_master is now explicitly denied to subscribers, as there is already
a lot of conditional logic there, we should consider to handle a logged in
subscriber separately. (This option is not exposed to subscribers anyway.)
See also 4967#c28533
MT#4967 "Unsteal" number on subadmin terminate

When a subscriberadmin terminates a subscriber, the alias_numbers should
move back to the subscriberadmin