Rich Text (etc) in @mention emails

at the moment when a user is mentioned in a comment they receive an email but any rich formatting, links or images don't come through in the same way they do on Loomio.

see here for preliminary mockups - my thoughts on what the email a user receives should look like.

Note
To support older mail services we include a text-only email.
The html emails are the richer ones and there are two of these depending on whether the comment was written with writ formatting on/off