Some live tasks make new elements available without requiring all
currently active ones to be completed first.
This adds the `req_activity_id` field to task activities which will mark
an element active if its required activity id is completed. If a valid
value is set then it's used instead of checking the current step.
The `step` field may still be set on rows with a valid `req_activity_id`
to specify its logical step and prevent later steps from becoming active
until completed. It's only ignored when deciding if the current element
is active.
The legacy task logic for unlocking activities was completely refactored
for this. A common method has been added so both zone and world can make
use of it to determine which elements are currently active. The previous
step system should remain unchanged.
The world logic for locking shared tasks when an element became active
did not account for "sequential" mode (all steps 0), unordered steps, or
gaps in step numbers. This also resolves that issue.
