Fix user resolution in 'Terms' aggregates widget

When a Terms aggregates widget with a *widget type User Checkbox is used in the default search, usernames are not translated to their first and last name when the username is prefixed with user:

How to reproduce:
In Studio Modeler:

• create a new document type named MyDoc extending Nothing:
  • in its schema mydoc add a field named user of type User/Group
  • in the Creation Layout tab, D&D the field mydoc:user in the form and make sure that the Widget Type is Single user/group suggestion and Use prefixed reference is set to Yes
• navigate to Listing & Views > Search and create the default search if not existing already
• in the search layout section, D&D an Terms aggregates widget and edit it:
  • set its property Field to mydoc:user
  • set its property Widget Type to User Checkbox
• deploy the Studio project to a Nuxeo Platform and start it
• log in
• create a document of type MyDoc in a workspace and set its property mydoc:user to Administrator. Make sure that the user Administrator has a first name and a last name
• navigate to the SEARCH main tab

Expected result: the aggregates for mydoc:user contains a checkbox with the first and last name of the Administrator user as a label
Actual result: the aggregates for mydoc:user contains a checkbox with the label user:Administrator

Workaround:

• Edit the Terms aggregate widget
• unfold the Custom properties configuration
• add a property with its key set to itemValue and its value:

  #{item.key.contains('user:') ? item.key.substring(5) : item.key}