Initial custom template for Missions.
If invalid user (such as a deleted/renamed userid), inform with message
that invalid user was found and to edit and re-add members, authors.

Fixes #61
 * Template checks if user members exist, and displays non-fatal error
   if there is problem such as if a user is deleted. Fixes #61

 * Mission reports custom template uses macro, for re-use in Mission
   Reports.

 * Checks if Mission Report not added, and provides hint.