Unsubscribe and Show Online links

Created by: artfulrobot

CiviMail offers tokens for unsubscribing and to show an online version of the message.

Mosaico has a built-in hard-coded part to its templates that introduce it's own custom tokens.

The Mosaico custom tokens are replaced by Mosaico for CiviMail-sent mailings, which is the intended use. However when using this extension to convert a Mosaico mailing to a Message Template these tokens remain and will not be intercepted by the Mosaico extension.

This requires some thought and planning. We should do what we can but also flag this in the README.