engine/forms:

Added optional parameters for performing a post-calc migration of
results.
engine/tasks:

Added post-calculation migration signal to calculation tasks.
engine/views:

Wire in the calc result migration parameters/etc. to the `run calc`
views.
engine/views:

Added some more 404 responses in places where data might be empty or
non-existent.
engine/views:

Added a TODO about limiting file download sizes.
engine/tests:

Added tests for several 404 cases and made existing tests work with
the updates to the task interfaces.