Allow Link Issues permission for users without agent license

Current Behavior
A user must have a Service Desk agent license in order to add a link from an issue in a Service Desk project to another issue.

• A user does not need an agent license to link from an issue in a Business or Software project to a Service Desk project. The link appears on both issues after being added.

"IGNORED_PERMISSIONS" list does not include LINK_ISSUES

Suggested Change
Remove veto/override for LINK_ISSUES so that this permission in project permission scheme applies to all users and so that users who are not agents can see Add Link option on issues in Service Desk projects.