Simplifies activity spawning when dependencies are conditionally set:
  activate(
    after_tag=some_value if some_condition else None,
  )
instead of having to do a ** dance.
All columns involved in dependency checking are declared NOT NULL, so providing
"None" and expecting a dependency to happen was already not working.
This change pushes this one step further by allowing activity
auto-validation also happen on these activities.
Also, simplify getOrderValidationText: avoid iterating on keys and then
retrieving values, use a list-comprehension, simplify condition.
Add a test for after_tag, also covering activity auto-validation.