Use library to derive plain text from HTML instead of using plain text content

[JayDaley]

There is at least one commonly used mail client that sends HTML mail using indentation for quoting, but then does not include that indentation as whitespace in the plain text content. See this message for an example.

To get around this, we could use a library that converts the HTML to plain text, for example the get_text() function in BeautifulSoup