WP Mailster v1.6.19

[Bug Fix] Add protection against CSRF attacks (kudos to Emanuele Barbeno)
[Bug Fix] Do not add mail recipients to queue when already moderated (approved) message gets approved again
[Bug Fix] Emails can have multiple attachments with the same name (not overwriting each other)
[Bug Fix] PHP8 compatibility work: do not use curly braces for index access in arrays in string