Choose default sender email address by envelope recipient

  - propagate the stored 'enVelopeto' to the citadel protocol as 'nvto' header; nvto may contain several recipients.
  - add handlers to read nvto in webcit
  - evaluate which should be the prefered chosen default sender while loading the GVEA Hash instead of while rendering the template
  - if we are replying to an email, base choice on the nvto-header if it exists.
  - in all other cases use the preference chosen in the prefs-screen