Deprecated mail.fromaddress, mail.from is Title <email@address>

Closes #4018

* cleaned up `mail_spec.js`
* deprecated `mail.fromaddress`
* implemented 'Blog title <email@address.com>' format with fallbacks
* added tests to deprecation and from address, made existing ones more robust
* moved domain intuit into its own module: `GhostMailer.getDomain()`