Feature Request: Add Dataverse support for customizing placeholder text used by the external controlled vocabulary functionality

[jggautier]

Overview of the Feature Request
Dataverse itself needs to able to support the customization of the placeholder text used by the external controlled vocabulary functionality, so that installations can more easily change the placeholder text, without having to fork the script used by the external controlled vocabulary functionality, and so that text can more easily be translated and displayed in other languages.

What kind of user is the feature intended for?
(Example users roles: API User, Curator, Depositor, Guest, Superuser, Sysadmin)
Superuser, Sysadmin

What inspired the request?
The placeholder text was adjusted when the external controlled vocabulary functionality was used to change metadata fields on Demo Dataverse (see IQSS/dataverse.harvard.edu#327).

In that GitHub issue, @qqmyers wrote that the "ability to customize the placeholder dynamically is supported by the scripts, but evidently not in Dataverse itself," and that Dataverse will need to support customization of the placeholder text too so that Dataverse installation admins don't need to fork the script used by the external controlled vocabulary functionality, and so that the text can more easily be translated and displayed in other languages.

Any open or closed issues related to this feature request?
IQSS/dataverse.harvard.edu#327
IQSS/dataverse.harvard.edu#326