s/rollback/rollback_unless_managed/

When creating a new SectionAsset model instance fails because of an
IntegrityError because there's already an asset assigned to the specified
section and container, the transaction needs to be rolled back.  However,
this is only needed for testing because normally autocommit mode is used.
Using django.db.transaction.rollback_unless_managed instead of
django.db.transaction.rollback seems to do the trick and only tries to
do a rollback if autocommit isn't being used.

Addresses #595
Refresh asset views after conflicting asset error

When the user tries to specify asset content for a section/container that
already has content assigned, likely because the builder is open in
multiple tabs/windows, refresh the section's assets collection and the
asset views to show the asset specified in the other window (see #535).

Also fix a bug (#696) where the asset views don't appear after switching
to a different workflow step and switching back to the build step.
Update changelog