FIX #10789 Add a warning if we add a contact that has a user

[FlorentPoinsaut]

FIX #10789 Add a warning if we add a contact that has a user

[eldy]

Can you check the user of the external user is correctly assigned to the project ? If not, this is the trouble.

Current rule to manage permission on project is:
Your user is assigned to the project, you can see the project.
Your user is not, you can't (except if you have the permission see all project even if not assigned).

So permission is managed by the user and only the users. Trying to introduce permissions based on other element than users is dangerous. it may generates a lose of control of the security. All the security layer must be based on the user and only the user.
So current behaviour is the expected feature.

it is clearthat being able to assign a contact to a project may let think we grant permission. But permission system is not handled by contacts, but by users and only users. So i recommend instead of this PR, to add a picto "warning, you must assign the user to allow access..." on the page to assign contact when we assign a contact (that is just an information) of an external user instead of assigning the user (that give the permission).

[FlorentPoinsaut]

OK @eldy I add a warning message :)