Added Useful links based on user type

[bigboss98]

As requested by IB, it is now possible to select who can see an useful link, maybe in future needs to be improved UI

[rbento1096]

@bigboss98 I have made some minor improvements which you can see in this commit

Namely:

• changed the type of visibleTo from string[] to AuthService[]
• changed the load() logic to start with EA and CO as default visibility. Also added a logic to filter any services that may be added to the visibleTo array but that no longer exist in the code.
• Changed the validate logic to throw an error if a service is not present (if in the future we remove one, users would have an error on save with no way to deselect the invalid auth service). Instead I changed it to throw an error in case the visibleTo array is empty. This can also be removed in case we want links only for admins.
• In the HTML I noticed we had ion-items inside an ion-item. This is not correct so I moved the item with label above the other two.
• Additionaly, I noticed the auth service type was hardcoded on the HTML. This isn't incorrect however it is better to move that logic inside an *ngFor that iterates the existing AuthServices enum.