Improve templateability

[jhoobergs]

• [x] Allow default values
• [ ] Allow optional description
• [x] Allow loading templated questions in exams:
  • [x] Need to set the path to the templated question
  • [x] Need to give values for all template fields (that don't have a default value)
  • [ ] Use this feature to support importing numbas exam with overwriting question variables
  • Also see the PS at https://github.com/m8rex/rumbas/issues/49#issuecomment-899331531

[jhoobergs]

Partially handled by #103 and #104