mfExtend() is a prevalent utility in MobileFrontend and MinervaNeue. In the former, ES6 is now available and canonical class extends are preferable to a custom utility. This is a task to continue the work in T220847 by converting Anchor to a class and dropping mfExtend() usage.

Anchor's parent is View and it's slim, so the work should be similar to that done for the BetaOptInPanel.

Acceptance criteria

• Anchor is a class that extends View.
• Anchor invokes View with the correct arguments via super.
• Properties needed during construction by the ancestors, at least View.defaults and View.template, remain on the prototype chain as they did for BetaOptInPanel.
• mfExtend() is removed and its overrides are moved to class properties and methods.
• If successful, a new task for further mfExtend() refactors is made. Maybe Button? Maybe something more aggressive that refactors a few tasks at once?