Introduce email settings action hooks

Sometimes id's desirable to customize the email settings screen for a custom email, or even for a core email. 

A few use cases for these hooks:

1. Display additional information about an email before the settings, below the description - like when is this email triggered - information, which may be programmatically generated by a plugin, and not that can easily passed in via the description.
2. Display template customization instructions after the settings table, before the template area.
3. Trigger custom action when a template is copied or deleted.