feat(AttributeLinkedSet): Allow for export in Spreadsheet HTML format

[Hipska]

Base information

Question	Answer
Related to a SourceForge thead / Another PR / Combodo ticket?	N/A
Type of change?	Enhancement

Objective

Exporting a LinkedSet attribute with spreadsheet HTML just returns the list of IDs of the linked objects, which is not very helpful for further processing.

Proposed solution

• Export the friendly names for the linked objects when localised, IDs otherwise.
• Add option to display the attribute as CSV in templates.

Checklist before requesting a review

• I have performed a self-review of my code
• I have tested all changes I made on an iTop instance
• I have added a unit test, otherwise I have explained why I couldn't
• Is the PR clear and detailed enough so anyone can understand digging in the code?