Extract out form change redirects into decorator

This is mostly common logic for update and create and it's slightly
confusing to mix into the actual update logic (the post to stagecraft)
Revert "Remove reference to Redis"

This reverts commit 5202f136a4886def5580928019fce9805e5bbf26.
Re-add Redis session

Cookies have a size limit of 4096 so reintroducing the Redis session to
allow larger dashboards to be edited.
Delete dashboard session on edit form

The 'pending_dashboard' session variable is used for passing saved state
from the create / update routes back to the form rendering routes. As
such it should only live for that single request. If the pending
dashboard stays in the session beyond that we risk showing unexpected
(and arguably incorrect) data in the form in certain situations. For
example, if a user goes back to the list page before saving and then
clicks back into the same dashboard the data shown would not be the data
that's stored in stagecraft but stale data from the session.