This is a follow up on #265

Here is what I am thinking:

1. adding more variable to the TemplatesCustomization class like this:

    def __init__(self, *args, **kwargs):
        for email_template, extension in self.files:
            self.__class__.variables[f"{email_template}_subject"] = ""
            self.__class__.variables[f"{email_template}_send_from"] = ""
            self.__class__.variables[f"{email_template}_cc_addresses"] = ""
        super().__init__(*args, **kwargs)

2. the subject would be a django template string, allowing things like {{ variable }} following the same provided vars as the ones in the email template
3. the send_from would have a datalist html widget with a few predefined choices (user_office, abuse, creator, server_email, etc) and allow custom entry.
4. the cc_addresses would allow multiple entries
5. for #3 and #4 we could also simply use variable resolution to allow {{ user_office_email }} for example.
6. for all the default values, I think the only way here would be to create a data migration that would add all the current send_from and subject and cc_addresses
7. this would be available in the "file & email templates" page
8. we would need a way to differentiate emails from other files/fragments (maybe using the extension email instead of html and having code to change it to html when loading the actual media file.