Add Item Prefix into E-mail Templates

I'm proposing to add "Item Prefix" field into E-mail Templates section.

This would be an unit config prefix, that will send e-mail using given e-mail template. In practice this would allow us to do more thorough e-mail template syntax validation for tags where unit config prefix is missing (e.g. <inp2:Field name="Title"/>) and is actually given at time, when e-mail is sent.