Add uniqueness rule to lnkContactToFunctionalCI

[Hipska]

While the UI for both FunctionalCI and Contact dictate there can be no duplicates for the linked list, it is still possible to create duplicates by using workarounds like for example import.

By adding a rule, this will be prevented and doesn't confuse users.

[Hipska]

I can also provide rules for other links if wanted

[piRGoif]

Technically this seems good I'll need to have the product team opinion on this though...

[Molkobain]

Pending after functional team review : "Datamodel changes are never simple, @etaloc will discuss it with you next week (you have a meeting)"

[Hipska]

Hmm, I forgot what the outcome of that conversation was. Could you ask @etaloc again please?

[piRGoif]

Setting this PR to reviewed on next functionnal review meeting (next week) : that will be a good place to ask Erwan :o)

[Hipska]

Discussed with @etaloc; The solution suggested by Combodo for 3.1 is implicit (when a link only has 2 external keys it is assumed to have uniqueness) while my solution in this PR is explicit and easier to override by customers. Internal discussion will need to be done on which path will be chosen..

[piRGoif]

Ok thanks :) Will be discussed during next functionnal review !

[Molkobain]

Put on "Pending Combodo update" as Erwan / Thomas discussion led to "wait for iTop 3.1 linkedset refactoring discussions"

[Hipska]

Solved by #416