UX of file/page link descriptions is inconsistent with fields in the modal

[GuySartorelli]

In #103 we introduced the following:

• LinkField does not display information about link the current content author does not have CanView rights on.

This means that for SiteTreeLink and FileLink, if the user doesn't have canView() permission for the page/file being linked to, there is a "Cannot view page" (or file) message displayed in the field.

This behaviour is not consistent with what happens in the modal:

• For a FileLink, the upload field is editable but shows no file inside it. A well-meaning content author might think there is no file for this link and upload a new file to it - which would override the file that was already there.
• For a SiteTreeLink, the tree dropdown field is editable and shows the name of the page - this means that the user is told "cannot view page", but they can view it if they just click through to the modal.

Expected outcomes

Option one

Change how these fields behave in framework/asset-admin when the record saved in them fails a canView() check.

Option two

Adjust the way we're handling these in linkfield to align with the behaviour that the fields in the modal provide