Folders with reserved names can not be deleted/renamed

If you create or import subfolders that have names such names as Sent and Trash, you can no longer delete and rename these folders.

Steps to reproduce:

• create two subfolders of the inbox with these names: Trash and Sent
• rename and/or delete these folders

Actual behaviour:

• the operation is not premised and the interface shows a non-speaking error

Expected behaviour:

• the operation is allowed asking for a confirmation before being performed

Internal Ref: http://helpdesk.nethesis.it/a/tickets/52190