Allow Custom Subject Title

[ItzNotABug]

Custom subjects could be like :

• {Post} • {Author Name}
• {Post} • {Primary Tag}
• {Post} • {Newsletter Name}
• {Post} • A custom message
• A custom message • {Post}

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Basically we can allow using the listed placeholders for subject, empty / default value should be just the Post name. Example: {{post_title}}, {{primary_tag}}, {{primary_author}}, {{newsletter_name}}, etc.