Add a tool to find duplicate URLs in learning objects

[benoitg]

It is generally not usefull to describe learning ressources multiple times (it is better to focus efforts to improve an existing description). It is however difficult to detect when this is done inadvertently. One way to do it for web based resources is to look dor duplicate URLs in the database, as these often (but not always) indicate a duplicated resource.

This feature should have two part:

1. A report of duplicate URLs, similar to the one used to find dead URLs.
2. A warning sent to the user when he tries to add a URL that is already part of the metadata of another learning resource.