Feature/822 addr verif credentials

I have created a new Address Verification Settings custom setting with Protected visibility. Before it had Public visibility, and it's not possible to change it. I deleted the old one. I hope this does not cause any issues, @jlantz.

Also, I have changed the Address Verification Settings page to stop displaying the Auth ID and Token, both as the user enters them, and in view mode.

I have not found any place where the settings are made available to the rest of the system through a global method (this was the third point for this item in the security review). @davidhabib or @kbromer, do you know of any?
Because I changed the property to be private. Initially I did that to obfuscate the ID and Token in the getter. However, that was causing issues with the "Test My Settings" functionality, so I ended up using a apex:outputText to hide them (see changes in STG_PanelAddrVerification.page).