Add `mail` action `mailmessage_html`

* also modify functions in `message` module and `mail` action to
  support creation of HTML code

* `mailmessage_html`:
  + sends mail as HTML + plain text
  + remote number is displayed as a link to either e.g.
    - `tel:+493012345678` (clickable on smartphones) or
    - `sip:username@siphost.net`