Activities/Identifiers: Action link text for identifiers should be different if this is the first identifier to be added

[dandineen]

Our 'Add another...' component pattern has different link text depending on whether the list of items - in this case identifiers - is empty or not.

If the Work identifiers section is empty then the action link should read: '(+) Add an identifier'.

The 'Add another...' component behaviour is defined in https://www.figma.com/file/FT9ayPqAt98wG4PBdBHrdR/Modal-windows-https-trello.com-c-Qej43BsL?node-id=863%3A508

UX/Product to define copy for all instances of the 'Add another' component and update the file accordingly.