ui: Improve display of long public key contents

by enabling vertical scrolling in key content's textarea element
ui: Ensure user is member to the created ip address project
ui: Introduce gateway input in network create overlay

the available options mimic the api behaviour as follows

- when dhcp option is not selected and thus no subnet will be
  automatically created, gateway value is not processed.
- when gateway type is set to `auto` no gateway_ip parameter is set in
  the POST request. API will automatically assign an available ip
  address for the subnet gateway.
- when gateway type is set to `none` gateway_ip paremeter is set to
  null. A subnet gets created with no gateway ip address.
- when gateway type is set to `manual`, an option only available when
  subnet type is not set to `auto`, the manually gateway input value is
  used in POST request. UI validates the valid form of the input value
  but it *does not* validate if provided ip address is within the range of
  the subnet cidr.