CRM-19690 - Task_AdhocMailing - Use highest-priority template type

CRM-19690 - Task_AdhocMailing - Use highest-priority template type

If you perform a contact search and create a new mailing, it would use
`template_type=traditional`, even if another template type had greater
priority. With this patch,  it respects the priority.

Note: I considered changing the default in Mailing.create API to always
match most-preferred template-type.  However, that would break some existing
API consumers (e.g.  headless consumers or ones who define their own UI).
For external API-based integrations, we should preserve the default
semantics of `body_text`/`body_html` by defaulting to
`template_type=traditional`.

The preference in CRM-19690 is that any use-case based on the screen
`civicrm/a/#/mailing/{id}` should have its default determined by weight.