Create 'link to content on all domains' permission in domain access linkit module

Problem/Motivation

The domain access linkit module currently has a permission that allows the user to be able to link to content on the domains they've been assigned to which is useful, but there are situations where it's useful for the user to be able to link to all domains without necessarily being assigned to those domains

Steps to reproduce

1. Set up a user and assign them to a sub domain
2. Create a piece of content on that sub domain and set up ckeditor with the linkit plugin
3. Search for content and see that they can only search for content within that sub domain

Proposed resolution

Create a new permission which allows the linking of content on all domains