LinkField honours Link permission

[maxime-rainville]

As the owner of a Silverstripe CMS project, I want link data to be access and updated only by authorised content author.

Acceptance criteria

• LinkField does not display information about link the current content author does not have CanView rights on.
• LinkField does not let you update Link data if you do not have CanEdit permission on the Link.
• LinkField does not let you delete Link data if you do not have CanDelete permission on the Link.
• LinkField does not let you create Link data if you do not have CanCreate permission on the Link.
• Any relations associated to a Link also honor access control restrictions (e.g: related page or file).
• Any APIs used to retrieve/update link data honours any relevant access control restriction.
• Toast errors should still be shown when the user does a request that return an HTTP error.
• UI buttons to create / edit / delete links should not show if the user fails corresponding permission checks

Excluded

• Readonly and hidden views when the user doesn't have the right update or view the link. That will come later.

Pull request

• ENH Tidy up permissions #141

[GuySartorelli]

PR merged