Add option to include author name in body of notification email

See the discussion on https://github.com/gitlabhq/gitlabhq/commit/ddd806c66f7b1b064243f81e36412ddbe557b8da (cc @huacnlee).

Some Exchange configurations don't allow applications like GitLab to override the sender on outbound email, which means the issue/MR/note author will not be anywhere in the notification email.

303f79f8 added "X wrote:" to every notification email, but this looked odd when the author name was already in the sender field.

I think we should make this configurable, as a fallback for those systems that can't use the sender to show the author.

cc @JobV