Fix FAQ/KB

[AdrienClairembault]

The FAQ / KB view does not seem to work properly.
You can see on the following images that the category tree indicate that some articles should be visible, however when you click on the category you find out that there is no valid articles:

The code handling this list was hard to read because it was handling two different cases (FAQ and KB) at the same time.

I've:

• Split and moved the code around to have proper KB / FAQ "code paths" which are easier to read and understand.
• Added a great number of tests which should handle every possible KB configuration.

Regarding the specific error in the images above, I think the is_faq => 1 condition was not properly applied for self-service users (it was thrown in an OR clause with other criteria that should only be used for the KB side, not the FAQ).
The new code handle it correctly as it separate KB and FAQ more cleanly, thus only applying the correct condition.

More details on the FAQ / KB expected restrictions can be found here : https://glpi-user-documentation.readthedocs.io/fr/latest/modules/tools/knowledgebase.html

Only public FAQ items are visible to users of simplified interface.
FAQ items are public to everyone (within their entity) that has access to read FAQs.
The additional visibility restrictions will have no affect in this case

Other elements are visible only to technicians via standard interface

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	!25026

[AdrienClairembault]

Wait before merge, I've got one final check to do following internal ref feedback

[AdrienClairembault]

Everything seems to be OK, the issue found in the internal ref was related to another issue directly in formcreator this time (see PR mentioned above).
Ready for merge :)