refs https://github.com/TryGhost/Team/issues/755
Make use of the new settings in the email template when `enableDeveloperExperiments` flag is enabled.
- added header image output if set
- hide all header output if both show publication title+icon are disabled
- hide individual header output for title and logo based on individual settings
- add left-align and serif classes to title based on individual settings
- hide feature image when disabled
