Upgrade email system within Acorn

One of the points of friction which prevents adoption of Acorn by new users is the deployment complexity and number of integrations to get up and running. To that end, it makes sense to remove the hard dependency that Acorn currently has on Mail Gorilla.

Development tasks:

• Upgrade mailcore and wgtmail to remove the dependency on Mail Gorilla (preserve Mail Gorilla backend so that it can continue to be used as an option, but the packages should not require it)
• Modify mailcore and wgtmail package to work with any mail backend
• Deprecate and remove mailmosaico in favor of the built-in email system (currently based on Wagtail StreamField)

Project Tracking

OP#58 OP#59