Support Custom Email Subjects

[ItzNotABug]

This PR adds the ability to use a custom subject for newsletter emails in the Newsletter Settings section.

Available placeholders -

{{post_title}}, {{primary_tag}}, {{primary_author}}, {{newsletter_name}}.

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Custom subjects could be like :

• {{post_title}} • #{{primary_tag}}
• {{post_title}} • {{primary_author}}
• {{post_title}} • {{newsletter_name}}
• {{post_title}} • A custom message
• A custom message • {{post_title}}

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Current limitations

• We only support a bullet point circle ('•') as a separator between the placeholder to avoid complexities while replacing them with actual values.