Send email on changed descriptions

When the initial description of issues are modified, no e-mail is send.

→ A e-mail with the new description content should be send. → In the bottom of the e-mail, a unified diff of the old and new description should be send.

And optional, but also useful: If comments in a issue are modified, a email with just the new comment should be send. e.g. declaring in a initial sentence (or in the subject), that the comment was adjusted.

A certain E-Mail header should indicate that this is a comment modification, so that sieve e-mail filters are able to flag them.