Mail templates

A mail template is created with --type mail and deployed like any other. Its index.html has to carry these macros:

• {{unsubscribeUrl}}: link to unsubscribe from the mails.
• {{seeInBrowserUrl}}: link to open the mail in the browser.
• {{mailFooter}}: the footer of the mail. It also has to be listed among the params in manifest.json, otherwise a campaign using the template cannot be saved.

The upload rejects a mail without {{unsubscribeUrl}} or {{seeInBrowserUrl}}, and template validate says so first.