CRM-15578 - Mailing API - Replace "preview_recipients" action with chained calls

The special-purpose API "Mailing.preview_recipients" is no longer necessary. Instead,
one can chain simpler APIs ("Mailing.create" + "MailingRecipients.get" + "force_rollback")
CRM-15578 - Mailing API - Extend test for "preview" action:

 * Assert subject, text, and html
 * Use force_rollback; assert no side-effects
 * Rename return properties for consistency (eg text => body_text)
crmApi - Use Angular's $q deferred; report errors correctly
CRM-15578 - crmMailing2 - Extract crmMailingMgr service
CRM-15578 - crmMailing2 - Add "preview.html"