* add a plain text email template
* use plain text email template for file based emails
* add config option to SMTP emails named disable_html_emails
* config option is a boolean that when set to true will only send plain text emails
* add docs for more complex SMTP notifier options
* update template
* add rfc1341 multipart logic to notifier
* check for errors after identity_verification
* * fix nil ptr
* go mod tidy
* remove needless checks
* * use multipart/atlernative instead
* * add rfc5322 compliant date header
* * fix linting issues
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