CRM-19690 - Declare Mailing.template_type, Mailing.template_options, Hook::mailingTem...

CRM-19690 - Declare Mailing.template_type, Mailing.template_options, Hook::mailingTemplateTypes.

This adds two new fields to the schema.

Template types can be declared (along with preferred editor) using
hook_civicrm_mailingTemplateTypes, e.g.

```
function mymod_civicrm_mailingTemplateTypes(&$types) {
  $types[] = array(
   'name' => 'moasico',
   'editorUrl' => '~/crmMosaico/EditMailingCtrl/mosaico.html',
  );
}
```

Note: This only stores data.  Other commits will make use of that data in
more meaningful ways.