Provide a very basic plain_text mail alternative

Converts the html mail to basic plain text

# Adopted from: https://www.netskin.com/posts/20230716-autogenerate-plain-email-version
# Creates a very simple plain text version of html mail. Does NOT care for conserving links or other styling elements
# but only retains the pure information and some layout (newlines)

HTML Mail in fixtures:

Plain Text Mail: