Refactor banner block around LinkField

[maxime-rainville]

Banner block has a weird BlockLinkField. LinkField is better.

Acceptance criteria

• BlockLinkField and related classes are deprecated in CMS 5 and remove in CMS 6.
• Developers can migrate their CMS 5 Data to LinkField in CMS 6.

[maxime-rainville]

I'm assuming there's no way this could be done cleanly in CMS 5, but I would be keen to see what option we have if any.

[maxime-rainville]

We want to validate if this will be supported in CMS 6 first.