[PLT-1015] Make all Mattermost system emails Multi-Part, with both a HTML and Plain Text version. (#5698)

* Implementation to Make all Mattermost system emails Multi-Part, with both a HTML and Plain Text version

* update lib

* update code per review

* update to use the mattermost repo

NOTICE.txt

@@ -4655,7 +4655,7 @@ MIT license
 This product contains a modified portion of 'html2text', an HTML to raw text converter by Carlos Tadeu Panato Junior, based on `html2text` by Jay Taylor.
 
 * HOMEPAGE
-  * https://github.com/cpanato/html2text
+  * https://github.com/mattermost/html2text
 
 * LICENSE