Allow alternative email templating systems

We've had a few recent discussions which touch on CiviMail templating, i.e.
1. In implementing fuller Mosaico support, we need hooks/APIs for tracking metadata about Mosaico mailings (eg https://github.com/veda-consulting/uk.co.vedaconsulting.mosaico/issues/72) .
2. In civicrm-dev, we discussed some of the limits in CiviMail's Smarty implementation and found that almost all ideas for improving the behavior raise problems with maintaining/migrating templates+mailings.

We can improve both situations by allowing mailings in CiviMail to track a template_type and template_options and load a suitable composition UI. (By way of comparison, consider how Drupal nodes are allowed to define different content formats.) This functionality should not be allowed to affect the default appearance or processing of existing mailings/templates, but it should allow an extension to hook in with alternatives.

Further technical details will be documented as part of the PR process.